Memory device, semiconductor device, and electronic device

ABSTRACT

An object is to provide a memory device which does not need a complex manufacturing process and whose power consumption can be suppressed, and a semiconductor device including the memory device. A solution is to provide a capacitor which holds data and a switching element which controls storing and releasing charge in the capacitor in a memory element. In the memory element, a phase-inversion element such as an inverter or a clocked inverter includes the phase of an input signal is inverted and the signal is output. For the switching element, a transistor including an oxide semiconductor in a channel formation region is used. In the case where application of a power supply voltage to the phase-inversion element is stopped, the data is stored in the capacitor, so that the data is held in the capacitor even when the application of the power supply voltage to the phase-inversion element is stopped.

TECHNICAL FIELD

The present invention relates to a memory device, a semiconductor deviceincluding the memory device, and an electronic device including thememory device.

BACKGROUND ART

A transistor including a semiconductor film which is formed over aninsulating surface is an essential semiconductor element for asemiconductor device. Since there is limitation on manufacture of atransistor in terms of allowable temperature limit of a substrate, atransistor including, in an active layer, amorphous silicon that can bedeposited at relatively low temperatures, polysilicon that can beobtained by crystallization with use of a laser beam or a catalyticelement, or the like is mainly used for a semiconductor display device.

In recent years, a metal oxide showing semiconductor characteristics,which is called an oxide semiconductor, has attracted attention as anovel semiconductor material having high mobility obtained bypolysilicon or microcrystalline silicon and having uniform elementcharacteristics obtained by amorphous silicon. The metal oxide is usedfor various applications; for example, indium oxide is a well-knownmetal oxide and used as a material of a transparent electrode includedin a liquid crystal display device or the like. Examples of such metaloxides showing semiconductor characteristics include tungsten oxide, tinoxide, indium oxide, and zinc oxide, and transistors in each of which achannel formation region is formed using such a metal oxide showingsemiconductor characteristics have been known (Patent Documents 1 and2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

DISCLOSURE OF INVENTION

By the way, a register, which is one of memory devices, is a logiccircuit generally including a logic element such as an inverter or aclocked inverter, and a switching element. Since the operation speed ofthe register is far higher than that of a random access memory (RAM)which is used as a main memory, the register is used in a processor inorder to temporally hold data for holding an arithmetic processing, aprogram execution state, or the like.

In FIG. 16A, one memory element included in a register in whichinverters are used is illustrated. A register 1300 illustrated in FIG.16A includes an inverter 1301, an inverter 1302, a switching element1303, and a switching element 1304. Input of a signal IN to an inputterminal of the inverter 1301 is controlled by the switching element1303. A potential of an output terminal of the inverter 1301 is suppliedto a circuit of a subsequent stage as a signal OUT. The output terminalof the inverter 1301 is connected to an input terminal of the inverter1302, and an output terminal of the inverter 1302 is connected to theinput terminal of the inverter 1301 via the switching element 1304.

When the switching element 1303 is turned off and the switching element1304 is turned on, a potential of the signal IN which is input via theswitching element 1303 is held in the register 1300.

A more specific circuit configuration of the register 1300 of FIG. 16Ais illustrated in FIG. 16B. The register 1300 illustrated in FIG. 16Bincludes the inverter 1301, the inverter 1302, the switching element1303, and the switching element 1304. The connection structure of thesecircuit elements are the same as that in FIG. 16A.

The inverter 1301 includes a p-channel transistor 1310 and an n-channeltransistor 1311 whose gate electrodes are connected to each other. Inaddition, the p-channel transistor 1310 and the n-channel transistor1311 are connected in series between a node to which a high-level powersupply potential VDD is supplied and a node to which a low-level powersupply potential VSS is supplied. In a similar manner, the inverter 1302includes a p-channel transistor 1312 and an n-channel transistor 1313whose gate electrodes are connected to each other. In addition, thep-channel transistor 1312 and the n-channel transistor 1313 areconnected in series between a node to which a high-level power supplypotential VDD is supplied and a node to which a low-level power supplypotential VSS is supplied.

The inverter 1301 illustrated in FIG. 16B operates such that one of thep-channel transistor 1310 and the n-channel transistor 1311 is turned onand the other is turned off according to the level of potentialssupplied to the gate electrodes thereof. Thus, current between the nodeto which the power supply potential VDD is supplied and the node towhich the power supply potential VSS is supplied should be ideally zero.However, actually a minute amount of OFF-state current flows in theOFF-state transistor; therefore, the current between the nodes can notbe zero. A similar phenomenon occurs in the inverter 1302; as a result,power is consumed in the register 1300 even in a hold state in whichdata is not written.

In the case of an inverter manufactured using bulk silicon for example,an OFF-state current of about 1 pA is generated at room temperature at avoltage between nodes of about 3 V, although it depends on the size of atransistor. The memory element illustrated in FIGS. 16A and 16B includestwo inverters: the inverter 1301 and the inverter 1302; therefore, anOFF-state current of about 2 pA is generated. In the case of a registerincluding about 10⁷ memory elements, the OFF-state current of the wholeregister is 20 μA. As the temperature of an IC chip in which theregister is provided becomes higher, the power consumption becomeslarger and the OFF-state current of the register becomes severalmilliamperes.

In order to suppress power consumption, a method for stopping the supplyof a power supply potential to a register has been suggested. In themethod, in the case where data needs to be held for a long time in aregister, the data is temporally transferred to a non-volatile memorydevice provided on the periphery of the register. This is because theregister is a volatile memory device in which data is erased when thesupply of a power supply potential is stopped. However, since such anon-volatile memory device is mainly formed using a magnetic element ora ferroelectric, the manufacturing process is complex.

In the case where the power supply is stopped for a long time, the datais transferred to a memory device such as a hard disk or a flash memoryand then the power supply can be stopped; however, in the case where thepower supply is stopped for a short time, such a memory device is notsuitable because a time for placing the data back is needed.

In view of the above-described problems, an object of one embodiment ofthe present invention is to provide a memory device which does not needa complex manufacturing process and whose power consumption can besuppressed, a semiconductor device including the memory device, and anelectronic device including the memory device. In particular, the objectof one embodiment of the present invention is to provide a memory devicewhose power consumption can be suppressed by stopping a power supply fora short time, a semiconductor device including the memory device, and anelectronic device including the memory device.

In a memory element including a logic element by which the phase of aninput signal is inverted and the signal is output (hereinafter, referredto as a phase-inversion element) such as an inverter or a clockedinverter, a capacitor which holds data and a switching element whichcontrols storing and releasing charge in the capacitor are provided. Forthe switching element, a transistor including an oxide semiconductor ina channel formation region is used.

Specifically, the memory element at least includes two phase-inversionelements, a capacitor, and a switching element which controls storingand releasing of charge in the capacitor. A signal including data thatis input to the memory element is supplied to an input terminal of afirst phase-inversion element. An output terminal of the firstphase-inversion element is connected to an input terminal of a secondphase-inversion element. An output terminal of the secondphase-inversion element is connected to the input terminal of the firstphase-inversion element. A potential of the output terminal of the firstphase-inversion element or the input terminal of the secondphase-inversion element is output to a memory element or another circuitof a subsequent stage as a signal.

Each of the phase-inversion elements has a structure in which at leastone p-channel transistor and at least one n-channel transistor whosegate electrodes are connected to each other are connected in seriesbetween a first node and a second node.

The capacitor is connected to a node to which the potential of theabove-described signal is supplied via the switching element so as tostore tha data of the signal that is input to the memory element asneeded.

In the state where a power supply voltage is applied between the firstnode and the second node, when the signal including the data is input tothe input terminal of the first phase-inversion element, the data isheld in the first phase-inversion element and the second phase-inversionelement. In the case where the application of the power supply voltagebetween the first node and the second node is stopped, before theapplication of the power supply voltage is stopped, the above-describedswitching element is turned on and the data of the signal is stored inthe capacitor. With the above-described structure, even when theapplication of the power supply voltage to at least one of thephase-inversion elements is stopped, data can be held in the memoryelement.

The channel formation region of the transistor used for theabove-described switching element includes a highly-purified oxidesemiconductor, and therefore the transistor has a characteristic ofextremely low OFF-state current.

The oxide semiconductor is a metal oxide showing semiconductorcharacteristics, and has high mobility obtained by microcrystalline orpolycrystalline silicon, and uniform element characteristics obtained byamorphous silicon. In addition, an oxide semiconductor (a purified OS)which is highly-purified by reduction of impurities such as moisture orhydrogen which serve as electron donors (donors) is an i-typesemiconductor (an intrinsic semiconductor) or extremely close to ani-type semiconductor (a substantially i-type semiconductor).Specifically, impurities such as moisture or hydrogen contained in theoxide semiconductor are removed so that the value of the hydrogenconcentration in the oxide semiconductor measured by secondary ion massspectrometry (SIMS) is less than or equal to 5×10¹⁹/cm³, preferably lessthan or equal to 5×10¹⁸/cm³, more preferably less than or equal to5×10¹⁷/cm³, even more preferably less than 1×10¹⁶/cm³. With theabove-described structure, the carrier density of an oxide semiconductorfilm, which can be measured by Hall effect measurement, can be less than1×10¹⁴ cm⁻³, preferably less than 1×10¹² cm⁻³, more preferably less than1×10¹¹ cm⁻³ that is less than or equal to measurement limit. That is,the carrier density of the oxide semiconductor film can be extremelyclose to zero. Furthermore, the band gap of the oxide semiconductor isgreater than or equal to 2 eV, preferably greater than or equal to 2.5eV, more preferably greater than or equal to 3 eV. Thus, with the use ofthe oxide semiconductor film which is highly purified by sufficientlyreducing the concentration of impurities such as moisture or hydrogen,the OFF-state current of the transistor can be reduced.

The analysis of the hydrogen concentration in the oxide semiconductorfilm and a conductive film is noted here. The hydrogen concentration inthe oxide semiconductor film and a conductive film is measured bysecondary ion mass spectrometry (SIMS). Because of the principle of theSIMS analysis, it is known to be difficult to obtain accurate data inthe proximity of a surface of a sample or in the proximity of aninterface between stacked films formed using different materials. Thus,in the case where the distribution of the hydrogen concentration in thethickness direction of a film is analyzed by SIMS, the average value ofthe hydrogen concentration in a region of the film where almost the samevalue can be obtained without significant variation is employed as thehydrogen concentration. Also, when the thickness of the film is small, aregion where almost the same value can be obtained cannot be found insome cases owing to the influence of the hydrogen concentration of anadjacent film. In this case, the maximal value or the minimal value ofthe hydrogen concentration in a region of the film is employed as thehydrogen concentration in the film. Furthermore, in the case where amountain-shaped peak indicating the maximal value and a valley-shapedpeak indicating the minimal value do not exist in any region of thefilm, the value of the inflection point is employed as the hydrogenconcentration.

Note that it is known that the oxide semiconductor film formed bysputtering or the like contains a large amount of impurities such asmoisture or hydrogen. Moisture or hydrogen easily forms a donor leveland thus serves as impurities in the oxide semiconductor. In oneembodiment of the present invention, in order to reduce impurities suchas moisture or hydrogen in the oxide semiconductor film, the oxidesemiconductor film is subjected to heat treatment in a reduced-pressureatmosphere, an inert gas atmosphere of nitrogen, a rare gas, or thelike, an oxygen gas atmosphere, or an ultra dry air atmosphere (themoisture amount is 20 ppm (−55° C. by conversion into a dew point) orless, preferably 1 ppm or less, more preferably 10 ppb or less, in thecase where measurement is performed by a dew point meter in a cavityring down laser spectroscopy (CRDS) method). The temperature range ofthe heat treatment is preferably higher than or equal to 500° C. andlower than or equal to 850° C. (or a temperature lower than or equal tothe strain point of a glass substrate), more preferably higher than orequal to 550° C. and lower than or equal to 750° C. Note that this heattreatment is performed at a temperature not exceeding the allowabletemperature limit of the substrate to be used. An effect of eliminationof moisture or hydrogen by the heat treatment has been confirmed bythermal desorption spectrometry (TDS).

Heat treatment in a furnace or a rapid thermal annealing method (RTAmethod) is used for the heat treatment. As the RTA method, a methodusing a lamp light source or a method in which short-time heat treatmentis performed on a substrate that is transferred into a heated gas can beemployed. With the use of the RTA method, the time necessary for theheat treatment can be shorter than 0.1 hour.

Specifically, the transistor including the highly-purified oxidesemiconductor film that is highly-purified by the above-described heattreatment as an active layer has extremely low OFF-state current.Specifically, even when an element has a channel width (W) of 1×10⁶ μmand a channel length (L) of 10 μm, OFF-state current (drain current whenthe voltage between a gate electrode and a source electrode is less thanor equal to 0 V) can be less than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A,at a voltage (drain voltage) between the source electrode and the drainelectrode of from 1 V to 10 V. Therefore, OFF-state current densitycorresponding to a numerical value which is calculated by dividing theOFF-state current by the channel width of the transistor is less than orequal to 100 zA/μm. According to measurement, it is further found that atransistor can have a much lower OFF-state current density, which isfrom 10 zA/μm to 100 zA/μm, at a voltage between a source electrode anda drain electrode of the transistor of 3 V. In the measurement, atransistor including a highly-purified oxide semiconductor film and a100-nm-thick gate insulating film was used for a switching element whichstores charge of a storage capacitor, and the OFF-state current of thetransistor was measured by a change in the amount of charge in thestorage capacitor per a unit hour. Thus, in the memory device which isone embodiment of the present invention, the OFF-state current densityof the transistor including the highly-purified oxide semiconductor filmas an active layer can be less than or equal to 100 zA/μm, preferablyless than or equal to 10 zA/μm, more preferably less than or equal to 1zA/μm. Accordingly, the transistor including the highly-purified oxidesemiconductor film as an active layer has far lower OFF-state currentthan a transistor including silicon having crystallinity.

In addition, a transistor including a highly-purified oxidesemiconductor shows almost no temperature dependence of OFF-statecurrent. One of the reasons is that the conductivity becomes extremelyclose to that of an intrinsic semiconductor, and that the Fermi level islocated in the middle of the forbidden band because the oxidesemiconductor is highly purified by removal of impurities which serve aselectron donors (donors) in the oxide semiconductor. Another reason isthat the oxide semiconductor has an energy gap of greater than or equalto 3 eV and includes very few thermally excited carriers. Still anotherreason for almost no temperature dependence is that the source electrodeand the drain electrode are in a degenerated state. The operation of thetransistor is mostly performed by carriers injected to the oxidesemiconductor from the degenerated source electrode, and the carrierdensity does not have temperature dependence; therefore, the temperaturedependence of OFF-state current is unlikely to be shown.

By using a transistor having the above-described structure as aswitching element which holds charge stored in a capacitor, leakagecurrent from the capacitor can be prevented; therefore, withoutapplication of a power supply voltage, data is not erased but can beheld. In the period when the data is held in the capacitor, the powersupply voltage is not necessarily applied to at least one of thephase-inversion elements; as a result, surplus power consumption due tothe OFF-state current of the transistor used for the phase-inversionelement can be reduced, and the power consumption of the memory deviceand further the whole semiconductor device including the memory devicecan be suppressed to be low.

Note that in the transistor used in the phase-inversion element, asemiconductor other than an oxide semiconductor can be used, such asamorphous silicon, microcrystalline silicon, polycrystalline silicon,single crystal silicon, amorphous germanium, microcrystalline germanium,polycrystalline germanium, or single crystal germanium. In addition, inthe above-described transistor, a thin semiconductor film or a bulksemiconductor substrate may be used. As long as a p-channel transistorincluding an oxide semiconductor film can be manufactured, all of thetransistors in the memory element can include the oxide semiconductorfilm as an active layer, so that the process can be simplified.

In one embodiment of the present invention, the application of a powersupply voltage to a memory element can be controlled with the use of atransistor including an oxide semiconductor in a channel formationregion. As described above, the transistor including an oxidesemiconductor in a channel formation region has a band gap of 3.0 eV to3.5 eV, which is about three times as large as that of silicon. Sincethe transistor including an oxide semiconductor in a channel formationregion has a high withstand voltage, by controlling the application of apower supply voltage to the memory element with the use of thetransistor, the reliability of a semiconductor device can be increased.

As the oxide semiconductor, an oxide of four metal elements such as anIN—Sn—Ga—Zn—O-based oxide semiconductor, an oxide of three metalelements such as an In—Ga—Zn—O-based oxide semiconductor, anIn—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxidesemiconductor, an oxide of two metal elements such as an In—Zn—O-basedoxide semiconductor, a Sn—Zn—O-based oxide semiconductor, anAl—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor,a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxidesemiconductor, or an In—Ga—O-based oxide semiconductor, an In—O-basedoxide semiconductor, a Sn—O-based oxide semiconductor, a Zn—O-basedoxide semiconductor, or the like can be used. Note that in thisspecification, for example, an In—Sn—Ga—Zn—O-based oxide semiconductormeans a metal oxide containing indium (In), tin (Sn), gallium (Ga), andzinc (Zn), and there is no particular limitation on the stoichiometriccomposition ratio. The above oxide semiconductors may include silicon.

Alternatively, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

With one embodiment of the present invention, it is possible to providea memory device that can suppress power consumption and a semiconductordevice including the memory device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a memory element.

FIG. 2 is a circuit diagram of the memory element.

FIG. 3 is a circuit diagram of a memory element.

FIG. 4 is a circuit diagram of the memory element.

FIG. 5 is a circuit diagram of a memory element.

FIG. 6 is a circuit diagram of the memory element.

FIGS. 7A to 7E are views illustrating a method for manufacturing amemory device.

FIGS. 8A to 8D are views illustrating the method for manufacturing amemory device.

FIGS. 9A and 9B are views illustrating the method for manufacturing amemory device.

FIGS. 10A to 10C are views illustrating the method for manufacturing amemory device.

FIGS. 11A and 11B are views illustrating the method for manufacturing amemory device.

FIGS. 12A to 12C are cross-sectional views of memory devices.

FIGS. 13A and 13B are views illustrating structures of memory devices.

FIGS. 14A and 14B are a cross-sectional view and a top view of atransistor.

FIGS. 15A to 15E are views illustrating a method for manufacturing amemory device.

FIGS. 16A and 16B are circuit diagrams of a conventional memory element.

FIG. 17 is a block diagram of a CPU in which a memory device is used.

FIGS. 18A to 18F are views illustrating structures of electronicdevices.

FIG. 19 is a cross-sectional view of a transistor including an oxidesemiconductor.

FIG. 20 is an energy band diagram (a schematic diagram) along A-A′section illustrated in FIG. 19.

FIG. 21A is a view illustrating a state where a positive voltage(V_(G)>0) is applied to a gate electrode (GE) and FIG. 21B is a viewillustrating a state where a negative voltage (V_(G)<0) is applied tothe gate electrode (GE).

FIG. 22 is a view showing relation between the vacuum level and the workfunction (φm) of a metal, and between the vacuum level and the electronaffinity (χ) of an oxide semiconductor.

FIG. 23 is a cross-sectional view of a memory device.

FIG. 24 is a timing chart showing the operation of a memory device.

FIG. 25 is a timing chart showing the operation of a memory device.

FIG. 26 is a timing chart showing the operation of a memory device.

FIG. 27 is a timing chart showing the operation of a memory device.

FIG. 28 is a timing chart showing the operation of a memory device.

FIG. 29 is a timing chart showing the operation of a memory device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Accordingly, the invention should not be construed asbeing limited to the description of the embodiments below.

The present invention includes in its category the following variouskinds of semiconductor devices in which a memory device can be used:integrated circuits such as microprocessors, image processing circuits,digital signal processors (DSPs), and large scale integrated circuits(LSIs) including microcontrollers, RF tags, and semiconductor displaydevices. In addition, the semiconductor display devices include thefollowing in its category: liquid crystal display devices,light-emitting devices in which a light-emitting element typified by anorganic light-emitting element (OLED) is provided for each pixel,electronic papers, digital micromirror devices (DMDs), plasma displaypanels (PDPs), field emission displays (FEDs), and other semiconductordisplay devices in which a circuit element using a semiconductor film isincluded in a driver circuit.

Embodiment 1

A memory device which is one embodiment of the present inventionincludes one or a plurality of memory elements capable of storing 1-bitdata. In FIG. 1, an example of a circuit diagram of a memory elementincluded in a memory device of the present invention is illustrated. Amemory element 100 illustrated in FIG. 1 at least includes a firstphase-inversion element 101 and a second phase-inversion element 102 bywhich the phase of an input signal is inverted and the signal is output,a switching element 103, a switching element 104, a capacitor 105, and acapacitor switching element 106.

A signal IN including data that is input to the memory element 100 issupplied to an input terminal of the first phase-inversion element 101via the switching element 103. An output terminal of the firstphase-inversion element 101 is connected to an input terminal of thesecond phase-inversion element 102. An output terminal of the secondphase-inversion element 102 is connected to the input terminal of thefirst phase-inversion element 101 via the switching element 104. Apotential of the output terminal of the first phase-inversion element101 or the input terminal of the second phase-inversion element 102 isoutput to a memory element or another circuit of a subsequent stage as asignal OUT.

Note that in FIG. 1, an example in which inverters are used as the firstphase-inversion element 101 and the second phase-inversion element 102is illustrated; however, a clocked inverter can also be used as thefirst phase-inversion element 101 or the second phase-inversion element102 besides the inverter.

The capacitor 105 is connected to an input terminal of the memoryelement 100, i.e., a node to which a potential of the signal IN issupplied, via the switching element 103 and the capacitor switchingelement 106 so that the data of the signal IN that is input to thememory element 100 can be stored as needed. Specifically, the capacitor105 is a condenser including a dielectric between a pair of electrodes.One of the electrodes is connected to the input terminal of the firstphase-inversion element 101 via the capacitor switching element 106. Theother of the electrodes is connected to a node to which a low-levelpower supply potential VSS or a fixed potential such as a groundpotential is supplied.

For the capacitor switching element 106, a transistor including ahighly-purified oxide semiconductor in a channel formation region isused.

Note that the memory element 100 may further include another circuitelement such as a diode, a resistor, or an inductance, as needed.

Next, an example of a more specific circuit diagram of the memoryelement of FIG. 1 is illustrated in FIG. 2. The memory element 100illustrated in FIG. 2 includes the first phase-inversion element 101,the second phase-inversion element 102, the switching element 103, theswitching element 104, the capacitor 105, and the capacitor switchingelement 106. The connection structure of these circuit elements are thesame as that in FIG. 1.

The first phase-inversion element 101 in FIG. 2 has a structure in whicha p-channel transistor 107 and an n-channel transistor 108 whose gateelectrodes are connected to each other are connected in series between afirst node to which a high-level power supply potential VDD is suppliedand a second node to which a low-level power supply potential VSS issupplied. Specifically, a source electrode of the p-channel transistor107 is connected to the first node to which the power supply potentialVDD is supplied, and a source electrode of the n-channel transistor 108is connected to the second node to which the power supply potential VSSis supplied. In addition, a drain electrode of the p-channel transistor107 is connected to a drain electrode of the n-channel transistor 108,and potentials of the two drain electrodes can be regarded as apotential of the output terminal of the first phase-inversion element101. In addition, potentials of the gate electrode of the p-channeltransistor 107 and the gate electrode of the n-channel transistor 108can be regarded as a potential of the input terminal of the firstphase-inversion element 101.

The second phase-inversion element 102 in FIG. 2 has a structure inwhich a p-channel transistor 109 and an n-channel transistor 110 whosegate electrodes are connected to each other are connected in seriesbetween the first node to which the high-level power supply potentialVDD is supplied and the second node to which the low-level power supplypotential VSS is supplied. Specifically, a source electrode of thep-channel transistor 109 is connected to the first node to which thepower supply potential VDD is supplied, and a source electrode of then-channel transistor 110 is connected to the second node to which thepower supply potential VSS is supplied. In addition, a drain electrodeof the p-channel transistor 109 is connected to a drain electrode of then-channel transistor 110, and potentials of the two drain electrodes canbe regarded as a potential of the output terminal of the secondphase-inversion element 102. In addition, potentials of the gateelectrode of the p-channel transistor 109 and the gate electrode of then-channel transistor 110 can be regarded as a potential of the inputterminal of the second phase-inversion element 102.

In FIG. 2, the case where a transistor is used for the switching element103 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 1 supplied to a gate electrode thereof. Inaddition, the case where a transistor is used for the switching element104 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 2 supplied to a gate electrode thereof.

Note that in FIG. 2, a structure in which each of the switching element103 and the switching element 104 includes only one transistor isillustrated; however, the present invention is not limited to thisstructure. In one embodiment of the present invention, the switchingelement 103 or the switching element 104 may include a plurality oftransistors. In the case where the plurality of transistors which serveas switching elements are included in the switching element 103 or theswitching element 104, the plurality of transistors may be connected toeach other in parallel, in series, or in combination of parallelconnection and series connection.

In FIG. 2, a transistor including an oxide semiconductor in a channelformation region is used for the capacitor switching element 106, andthe switching of the transistor is controlled by a signal Sig 3 suppliedto a gate electrode thereof. Since the transistor used for the capacitorswitching element 106 includes a highly-purified oxide semiconductor ina channel formation region, OFF-state current is extremely low asdescribed above.

Note that in FIG. 2, a structure in which the capacitor switchingelement 106 includes only one transistor is illustrated; however, thepresent invention is not limited to this structure. In one embodiment ofthe present invention, the capacitor switching element 106 may include aplurality of transistors. In the case where the plurality of transistorswhich serve as switching elements are included in the capacitorswitching element 106, the plurality of transistors may be connected toeach other in parallel, in series, or in combination of parallelconnection and series connection.

Note that in this specification, the state in which the transistors areconnected to each other in series means the state in which only one of asource electrode and a drain electrode of a first transistor isconnected to only one of a source electrode and a drain electrode of asecond transistor. In addition, the state in which the transistors areconnected to each other in parallel means the state in which one of asource electrode and a drain electrode of a first transistor isconnected to one of a source electrode and a drain electrode of a secondtransistor and the other of the source electrode and the drain electrodeof the first transistor is connected to the other of the sourceelectrode and the drain electrode of the second transistor.

In one embodiment of the present invention, at least a transistor usedfor a switching element in the capacitor switching element 106 mayinclude a highly-purified oxide semiconductor in a channel formationregion. Accordingly, a transistor used for the first phase-inversionelement 101, the second phase-inversion element 102, the switchingelement 103, or the switching element 104 can include a semiconductorother than an oxide semiconductor, such as amorphous silicon,microcrystalline silicon, polycrystalline silicon, single crystalsilicon, amorphous germanium, microcrystalline germanium,polycrystalline germanium, or signal crystal germanium. In addition, inthe above-described transistor, a thin semiconductor film or a bulksemiconductor substrate may be used. As long as a p-channel transistorincluding an oxide semiconductor film can be manufactured, all of thetransistors in the memory element can include the oxide semiconductorfilm as an active layer, so that the process can be simplified.

Note that “connection” in this specification means electrical connectionand corresponds to the state in which current, voltage, or potential canbe supplied, applied, or conducted. Therefore, the state of connectiondoes not always mean a state of direct connection, but includes in itscategory a state of indirect connection via a circuit element such as awiring, a resistor, a diode, or a transistor, in which current, voltage,or potential can be supplied, applied, or conducted.

Note also that even when a circuit diagram shows independent componentsas if they are connected to each other, there is a case in which oneconductive film has functions of a plurality of components such as acase in which part of a wiring also functions as an electrode. The“connection” in this specification includes in its category such a casein which one conductive film has functions of a plurality of components.

The “source electrode” and the “drain electrode” included in thetransistor interchange with each other depending on the polarity of thetransistor or difference between the levels of potentials supplied tothe respective electrodes. In general, in an n-channel transistor, anelectrode to which a lower potential is supplied is called a sourceelectrode, and an electrode to which a higher potential is supplied iscalled a drain electrode. Further, in a p-channel transistor, anelectrode to which a lower potential is supplied is called a drainelectrode, and an electrode to which a higher potential is supplied iscalled a source electrode. In this specification, for convenience,although connection relation of the transistor is described assumingthat the source electrode and the drain electrode are fixed in somecases; however, actually, the names of the source electrode and thedrain electrode interchange with each other depending on the relationbetween the potentials.

Next, an example of the operation of the memory element illustrated inFIG. 1 will be described.

First, in writing data, the switching element 103 is turned on, theswitching element 104 is turned off, and the capacitor switching element106 is turned off. Then, the power supply potential VDD is supplied tothe first node and the power supply potential VSS is supplied to thesecond node, whereby a power supply voltage is applied between the firstnode and the second node. A potential of the signal IN supplied to thememory element 100 is supplied to the input terminal of the firstphase-inversion element 101 via the switching element 103, whereby thepotential of the output terminal of the first phase-inversion element101 is a phase-inverted potential of the signal IN. Then, the switchingelement 104 is turned on and the input terminal of the firstphase-inversion element 101 is connected to the output terminal of thesecond phase-inversion element 102, whereby data is written into thefirst phase-inversion element 101 and the second phase-inversion element102.

Next, in the case where the input data is held in the firstphase-inversion element 101 and the second phase-inversion element 102,in the state where the switching element 104 remains in an ON state andthe capacitor switching element 106 remains in an OFF state, theswitching element 103 is turned off. By turning off the switchingelement 103, the input data is held in the first phase-inversion element101 and the second phase-inversion element 102. At this time, the powersupply potential VDD is supplied to the first node and the power supplypotential VSS is supplied to the second node, whereby the state in whichthe power supply voltage is applied between the first node and thesecond node is maintained.

The potential of the output terminal of the first phase-inversionelement 101 reflects the data held in the first phase-inversion element101 and the second phase-inversion element 102. Therefore, by readingout the potential, the data can be read out from the memory element 100.

Note that in order to reduce power consumption in holding the data, inthe case where the input data is held in the capacitor 105, first, theswitching element 103 is turned off, the switching element 104 is turnedon, and the capacitor switching element 106 is turned on. Then, via thecapacitor switching element 106, charge with an amount corresponding tothe value of the data held in the first phase-inversion element 101 andthe second phase-inversion element 102 is stored in the capacitor 105,whereby the data is written into the capacitor 105. After the data isstored in the capacitor 105, the capacitor switching element 106 isturned off, whereby the data stored in the capacitor 105 is held. Afterturning off the capacitor switching element 106, for example, the powersupply potential VSS is supplied to each of the first node and thesecond node so that the nodes have equal potentials, and the applicationof the power supply voltage between the first node and the second nodeis stopped. Note that after the data is stored in the capacitor 105, theswitching element 104 may be turned off In such a manner, in the casewhere the input data is held in the capacitor 105, the application ofthe power supply voltage between the first node and the second node isunnecessary; therefore, the OFF-state current flowing between the firstnode and the second node via the p-channel transistor 107 and then-channel transistor 108 which are included in the first phase-inversionelement 101, or via the p-channel transistor 109 and the n-channeltransistor 110 which are included in the second phase-inversion element102 can be extremely close to zero. As a result, power consumption dueto the OFF-state current of the memory element in holding the data canbe significantly reduced, and the power consumption of the memory deviceand further the whole semiconductor device including the memory devicecan be suppressed to be low.

Since the transistor used for the capacitor switching element 106includes a highly-purified oxide semiconductor in a channel formationregion, OFF-state current density can be less than or equal to 100zA/μm, preferably less than or equal to 10 zA/μm, more preferably lessthan or equal to 1 zA/μm. Accordingly, the transistor including thehighly-purified oxide semiconductor film as an active layer has farlower OFF-state current than a transistor including silicon havingcrystallinity. As a result, when the capacitor switching element 106 forwhich the transistor is used is in an OFF state, charge stored in thecapacitor 105 is hardly released; therefore, the data is held.

In the case where the data stored in the capacitor 105 is read out, theswitching element 103 is turned off. Then, the power supply potentialVDD is again supplied to the first node and the power supply potentialVSS is again supplied to the second node, whereby the power supplyvoltage is applied between the first node and the second node. Then, byturning on the capacitor switching element 106, the signal OUT having apotential that reflects the data can be read out from the memory element100.

Next, FIG. 24 is an example of a timing chart in the case where thesupply of the power supply potential VDD is not stopped in the circuitillustrated in FIG. 2. When the signal Sig 1 is set to a low level andthe signal Sig 2 is set to a high level, the signal IN is blocked, and afeedback loop is formed, and the state can be held. When the signal Sig1 is set to a high level again and the signal Sig 2 is a set to a lowlevel, the signal IN is input and output via the first phase-inversionelement 101. At this time, the signal Sig 3 is set to a low level.

FIG. 25 is an example of a timing chart in the case where the supply ofthe power supply potential VDD is stopped in the circuit of FIG. 2. Whenthe signal Sig 1 is set to a low level and the signal Sig 2 is set to ahigh level, the signal IN is blocked, and a feedback loop is formed, andthe state can be held. After that, the signal Sig 3 is set to a highlevel, whereby the capacitor 105 stores the data (in FIG. 25, this stateis shown as a high level). Even when the supply of the power supplypotential VDD is stopped after that, a potential of the capacitor 105 isheld. After that, when the power supply potential VDD is supplied andthe signal Sig 3 is set to a high level again, the potential of thecapacitor 105 is output via the first phase-inversion element 101 (inFIG. 25, this state is shown as a low level).

Embodiment 2

In this embodiment, another example of a memory element included in amemory device of the present invention will be described. In FIG. 3, acircuit diagram of the memory element of this embodiment is illustratedas an example.

A memory element 200 illustrated in FIG. 3 at least includes a firstphase-inversion element 201, a second phase-inversion element 202, and athird phase-inversion element 207 by which the phase of an input signalis inverted and the signal is output, a switching element 203, aswitching element 204, a switching element 208, a switching element 209,a capacitor 205, and a capacitor switching element 206.

A signal IN including data that is input to the memory element 200 issupplied to an input terminal of the first phase-inversion element 201via the switching element 203. An output terminal of the firstphase-inversion element 201 is connected to an input terminal of thesecond phase-inversion element 202. An output terminal of the secondphase-inversion element 202 is connected to the input terminal of thefirst phase-inversion element 201 via the switching element 204. Apotential of the output terminal of the first phase-inversion element201 or the input terminal of the second phase-inversion element 202 isoutput to a memory element or another circuit of a subsequent stage as asignal OUT via the switching element 208.

The capacitor 205 is connected to an input terminal of the memoryelement 200, i.e., a node to which a potential of the signal IN issupplied, via the capacitor switching element 206 so that the data ofthe signal IN that is input to the memory element 200 can be stored asneeded. Specifically, the capacitor 205 is a condenser including adielectric between a pair of electrodes. One of the electrodes isconnected to the node to which the potential of the signal IN issupplied via the capacitor switching element 206. The other of theelectrodes is connected to a node to which a low-level power supplypotential VSS or a fixed potential such as a ground potential issupplied.

In addition, the one of the electrodes of the capacitor 205 is connectedto an input terminal of the third phase-inversion element 207. Apotential of an output terminal of the third phase-inversion element 207is output via the switching element 209 to a memory element or anothercircuit of a subsequent stage as a signal OUT.

Note that in FIG. 3, an example in which inverters are used as the firstphase-inversion element 201, the second phase-inversion element 202, andthe third phase-inversion element 207 is illustrated; however, a clockedinverter can also be used as the first phase-inversion element 201, thesecond phase-inversion element 202, or the third phase-inversion element207 besides the inverter.

For the capacitor switching element 206, a transistor including ahighly-purified oxide semiconductor in a channel formation region isused.

Note that the memory element 200 may further include another circuitelement such as a diode, a resistor, or an inductance, as needed.

Next, an example of a more specific circuit diagram of the memoryelement of FIG. 3 is illustrated in FIG. 4. The memory element 200illustrated in FIG. 4 at least includes the first phase-inversionelement 201, the second phase-inversion element 202, the thirdphase-inversion element 207, the switching element 203, the switchingelement 204, the switching element 208, the switching element 209, thecapacitor 205, and the capacitor switching element 206. The connectionstructure of these circuit elements are the same as that in FIG. 3.

The first phase-inversion element 201 in FIG. 4 has a structure in whicha p-channel transistor 210 and an n-channel transistor 211 whose gateelectrodes are connected to each other are connected in series between afirst node to which a high-level power supply potential VDD is suppliedand a second node to which a low-level power supply potential VSS issupplied. Specifically, a source electrode of the p-channel transistor210 is connected to the first node to which the power supply potentialVDD is supplied, and a source electrode of the n-channel transistor 211is connected to the second node to which the power supply potential VSSis supplied. In addition, a drain electrode of the p-channel transistor210 is connected to a drain electrode of the n-channel transistor 211,and potentials of the two drain electrodes can be regarded as apotential of the output terminal of the first phase-inversion element201. In addition, potentials of the gate electrode of the p-channeltransistor 210 and the gate electrode of the n-channel transistor 211can be regarded as a potential of the input terminal of the firstphase-inversion element 201.

The second phase-inversion element 202 in FIG. 4 has a structure inwhich a p-channel transistor 212 and an n-channel transistor 213 whosegate electrodes are connected to each other are connected in seriesbetween the first node to which the high-level power supply potentialVDD is supplied and the second node to which the low-level power supplypotential VSS is supplied. Specifically, a source electrode of thep-channel transistor 212 is connected to the first node to which thepower supply potential VDD is supplied, and a source electrode of then-channel transistor 213 is connected to the second node to which thepower supply potential VSS is supplied. In addition, a drain electrodeof the p-channel transistor 212 is connected to a drain electrode of then-channel transistor 213, and potentials of the two drain electrodes canbe regarded as a potential of the output terminal of the secondphase-inversion element 202. In addition, potentials of the gateelectrode of the p-channel transistor 212 and the gate electrode of then-channel transistor 213 can be regarded as a potential of the inputterminal of the second phase-inversion element 202.

The third phase-inversion element 207 in FIG. 4 has a structure in whicha p-channel transistor 214 and an n-channel transistor 215 whose gateelectrodes are connected to each other are connected in series between athird node to which the high-level power supply potential VDD issupplied and a fourth node to which the low-level power supply potentialVSS is supplied. Specifically, a source electrode of the p-channeltransistor 214 is connected to the third node to which the power supplypotential VDD is supplied, and a source electrode of the n-channeltransistor 215 is connected to the fourth node to which the power supplypotential VSS is supplied. In addition, a drain electrode of thep-channel transistor 214 is connected to a drain electrode of then-channel transistor 215, and potentials of the two drain electrodes canbe regarded as a potential of the output terminal of the thirdphase-inversion element 207. In addition, potentials of the gateelectrode of the p-channel transistor 214 and the gate electrode of then-channel transistor 215 can be regarded as a potential of the inputterminal of the third phase-inversion element 207.

Note that the first node and the third node may be electricallyconnected to each other as one node. Note also that the second node andthe fourth node may be electrically connected to each other as one node.

In FIG. 4, the case where a transistor is used for the switching element203 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 1 supplied to a gate electrode thereof. Inaddition, the case where a transistor is used for the switching element204 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 2 supplied to a gate electrode thereof.Further, the case where a transistor is used for the switching element209 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 4 supplied to a gate electrode thereof.

Note that in FIG. 4, a structure in which each of the switching element203, the switching element 204, and the switching element 209 includesonly one transistor is illustrated; however, the present invention isnot limited to this structure. In one embodiment of the presentinvention, the switching element 203, the switching element 204, or theswitching element 209 may include a plurality of transistors. In thecase where the plurality of transistors which serve as switchingelements are included in the switching element 203, the switchingelement 204, or the switching element 209, the plurality of transistorsmay be connected to each other in parallel, in series, or in combinationof parallel connection and series connection.

In FIG. 4, a transistor including an oxide semiconductor in a channelformation region is used for the capacitor switching element 206, andthe switching of the transistor is controlled by a signal Sig 3 suppliedto a gate electrode thereof. Since the transistor used for the capacitorswitching element 206 includes a highly-purified oxide semiconductor ina channel formation region, OFF-state current is extremely low asdescribed above.

Note that in FIG. 4, a structure in which the capacitor switchingelement 206 includes only one transistor is illustrated; however, thepresent invention is not limited to this structure. In one embodiment ofthe present invention, the capacitor switching element 206 may include aplurality of transistors. In the case where the plurality of transistorswhich serve as switching elements are included in the capacitorswitching element 206, the plurality of transistors may be connected toeach other in parallel, in series, or in combination of parallelconnection and series connection.

In one embodiment of the present invention, at least a transistor usedfor a switching element in the capacitor switching element 206 mayinclude a highly-purified oxide semiconductor in a channel formationregion. Accordingly, a transistor used for the first phase-inversionelement 201, the second phase-inversion element 202, the thirdphase-inversion element 207, the switching element 203, the switchingelement 204, the switching element 208, or the switching element 209 caninclude a semiconductor other than an oxide semiconductor, such asamorphous silicon, microcrystalline silicon, polycrystalline silicon,single crystal silicon, amorphous germanium, microcrystalline germanium,polycrystalline germanium, or signal crystal germanium. In addition, inthe above-described transistor, a thin semiconductor film or a bulksemiconductor substrate may be used. As long as a p-channel transistorincluding an oxide semiconductor film can be manufactured, all of thetransistors in the memory element can include the oxide semiconductorfilm as an active layer, so that the process can be simplified.

Next, an example of the operation of the memory element illustrated inFIG. 3 will be described.

First, in writing data, the switching element 203 is turned on, theswitching element 204 is turned off, the switching element 208 is turnedoff, the switching element 209 is turned off, and the capacitorswitching element 206 is turned on. Then, the power supply potential VDDis supplied to the first node and the power supply potential VSS issupplied to the second node, whereby a power supply voltage is appliedbetween the first node and the second node. A potential of the signal INsupplied to the memory element 200 is supplied to the input terminal ofthe first phase-inversion element 201 via the switching element 203,whereby the potential of the output terminal of the firstphase-inversion element 201 is a phase-inverted potential of the signalIN. Then, the switching element 204 is turned on and the input terminalof the first phase-inversion element 201 is connected to the outputterminal of the second phase-inversion element 202, whereby data iswritten into the first phase-inversion element 201 and the secondphase-inversion element 202.

In writing the data, via the capacitor switching element 206, chargewith an amount corresponding to the value of the data of the signal INis stored in the capacitor 205, whereby the data is written also intothe capacitor 205.

Note that in writing the data, application of a power supply voltagebetween the third node and the fourth node is unnecessary. Therefore,for example, the power supply potential VSS is supplied to each of thethird node and the fourth node so that the nodes have equal potentials.

Next, in the case where the input data is held in the firstphase-inversion element 201 and the second phase-inversion element 202,in the state where the switching element 204 remains in an ON state, theswitching element 208 remains in an OFF state, and the switching element209 remains in an OFF state, the switching element 203 is turned off andthe capacitor switching element 206 is turned off. By turning off theswitching element 203, the input data is held in the firstphase-inversion element 201 and the second phase-inversion element 202.At this time, the power supply potential VDD is supplied to the firstnode and the power supply potential VSS is supplied to the second node,whereby the state in which the power supply voltage is applied betweenthe first node and the second node is maintained.

In addition, by turning off the capacitor switching element 206, thedata written into the capacitor 205 is also held.

The potential of the output terminal of the first phase-inversionelement 201 reflects the data held in the first phase-inversion element201 and the second phase-inversion element 202. Therefore, by readingout the potential by turning on the switching element 208, the data canbe read out from the memory element 200.

Note that in order to reduce power consumption in holding the data, inthe case where the input data is held only in the capacitor 205, forexample, the power supply potential VSS is supplied to each of the firstnode and the second node so that the nodes have equal potentials, andthe application of the power supply voltage between the first node andthe second node is stopped. When the application of the power supplyvoltage between the first node and the second node is stopped, the dataheld in the first phase-inversion element 201 and the secondphase-inversion element 202 are erased, but the data written in thecapacitor 205 remains to be held.

In such a manner, in the case where the input data is held in thecapacitor 205, the application of the power supply voltage between thefirst node and the second node is unnecessary; therefore, via thep-channel transistor 210 and the n-channel transistor 211 which areincluded in the first phase-inversion element 201, or via the p-channeltransistor 212 and the n-channel transistor 213 which are included inthe second phase-inversion element 202, the OFF-state current flowingbetween the first node and the second node can be extremely close tozero. As a result, power consumption due to the OFF-state current of thememory element in holding the data can be significantly reduced, and thepower consumption of the memory device and further the wholesemiconductor device including the memory device can be suppressed to below.

In addition, in the case where the input data is held in the capacitor205, the application of the power supply voltage between the third nodeand the fourth node is unnecessary. Therefore, via the p-channeltransistor 214 and the n-channel transistor 215 which are included inthe third phase-inversion element 207, the OFF-state current flowingbetween the third node and the fourth node can be extremely close tozero. As a result, power consumption due to the OFF-state current of thememory element in holding the data can be significantly reduced, and thepower consumption of the memory device and further the wholesemiconductor device including the memory device can be suppressed to below.

Since the transistor used for the capacitor switching element 206includes a highly-purified oxide semiconductor in a channel formationregion, OFF-state current density can be less than or equal to 100zA/μm, preferably less than or equal to 10 zA/μm, more preferably lessthan or equal to 1 zA/μm. Accordingly, the transistor including thehighly-purified oxide semiconductor film as an active layer has farlower OFF-state current than a transistor including silicon havingcrystallinity. As a result, when the capacitor switching element 206 forwhich the transistor is used is in an OFF state, charge stored in thecapacitor 205 is hardly released; therefore, the data is held.

In the case where the data stored in the capacitor 205 is read out, thepower supply potential VDD is supplied to the third node and the powersupply potential VSS is supplied to the fourth node, whereby the powersupply voltage is applied between the third node and the fourth node.When the power supply voltage is applied between the third node and thefourth node, the output terminal of the third phase-inversion element207 is supplied with a phase-inversion potential of the potential of theinput terminal thereof. Note that the input terminal of the thirdphase-inversion element 207 is supplied with a potential having a levelcorresponding to the amount of charge stored in the capacitor 205; thus,a potential of the output terminal thereof reflects the data. Thus, byturning on the switching element 209, the signal OUT having a potentialthat reflects the data can be read out from the memory element 200.

FIG. 26 is an example of a timing chart in the case where the supply ofthe power supply potential VDD is not stopped in the circuit of FIG. 4.When the signal Sig 1 is set to a low level and the signal Sig 2 is setto a high level, the signal IN is blocked, and a feedback loop isformed, and the state can be held. When the signal Sig 1 is set to ahigh level again and the signal Sig 2 is a set to a low level, thesignal IN is input and output via the first phase-inversion element 201.At this time, the signal Sig 3 and the signal Sig 4 are set to a lowlevel, and a signal Sig 5 is set to a high level.

FIG. 27 is an example of a timing chart in the case where the supply ofthe power supply potential VDD is stopped in the circuit of FIG. 4. Whenthe signal Sig 1 is set to a low level and the signal Sig 2 is set to ahigh level, the signal IN is blocked, and a feedback loop is formed, andthe state can be held. On the other hand, the signal Sig 3 is set to ahigh level, whereby the capacitor 205 stores the data (in FIG. 27, thisstate is shown as a low level). Even when the supply of the power supplypotential VDD is stopped after that, a potential of the capacitor 205 isheld. After that, when the power supply potential VDD is supplied andthe signal Sig 5 and the signal Sig 3 are set to a low level and thesignal Sig 4 is set to a high level, the potential of the capacitor 205is output via the third phase-inversion element 207 (in FIG. 27, thisstate is shown as a high level).

This embodiment can be implemented by being combined as appropriate withthe above-described embodiment.

Embodiment 3

In this embodiment, another example of a memory element included in amemory device of the present invention will be described. In FIG. 5, acircuit diagram of the memory element of this embodiment is illustratedas an example.

A memory element 300 illustrated in FIG. 5 at least includes a firstphase-inversion element 301 and a second phase-inversion element 302 bywhich the phase of an input signal is inverted and the signal is output,a switching element 303, a switching element 304, a capacitor 305, acapacitor switching element 306, a capacitor 307, and a capacitorswitching element 308.

A signal IN including data that is input to the memory element 300 issupplied to an input terminal of the first phase-inversion element 301via the switching element 303. An output terminal of the firstphase-inversion element 301 is connected to an input terminal of thesecond phase-inversion element 302. An output terminal of the secondphase-inversion element 302 is connected to the input terminal of thefirst phase-inversion element 301 via the switching element 304. Apotential of the output terminal of the first phase-inversion element301 or the input terminal of the second phase-inversion element 302 isoutput to a memory element or another circuit of a subsequent stage as asignal OUT.

The capacitor 305 is connected to an input terminal of the memoryelement 300, i.e., a node to which a potential of the signal IN issupplied, via the switching element 303 and the capacitor switchingelement 306 so that the data of the signal IN that is input to thememory element 300 can be stored as needed. Specifically, the capacitor305 is a condenser including a dielectric between a pair of electrodes.One of the electrodes is connected to the input terminal of the firstphase-inversion element 301 via the capacitor switching element 306. Theother of the electrodes is connected to a node to which a low-levelpower supply potential VSS or a fixed potential such as a groundpotential is supplied.

In a manner similar to that of the capacitor 305, the capacitor 307 isconnected to an input terminal of the memory element 300, i.e., a nodeto which a potential of the signal IN is supplied, via the switchingelement 303, the first phase-inversion element 301, and the capacitorswitching element 308 so that the data of the signal IN input to thememory element 300 can be stored as needed. Specifically, the capacitor307 is a condenser including a dielectric between a pair of electrodes.One of the electrodes is connected to the output terminal of the firstphase-inversion element 301 via the capacitor switching element 308. Theother of the electrodes is connected to a node to which a low-levelpower supply potential VSS or a fixed potential such as a groundpotential is supplied.

Note that in FIG. 5, an example in which inverters are used as the firstphase-inversion element 301 and the second phase-inversion element 302is illustrated; however, a clocked inverter can also be used as thefirst phase-inversion element 301 or the second phase-inversion element302 besides the inverter.

For each of the capacitor switching element 306 and the capacitorswitching element 308, a transistor including a highly-purified oxidesemiconductor in a channel formation region is used.

Note that the memory element 300 may further include another circuitelement such as a diode, a resistor, or an inductance, as needed.

Next, an example of a more specific circuit diagram of the memoryelement of FIG. 5 is illustrated in FIG. 6. The memory element 300illustrated in FIG. 6 at least includes the first phase-inversionelement 301, the second phase-inversion element 302, the switchingelement 303, the switching element 304, the capacitor 305, the capacitorswitching element 306, the capacitor 307, and the capacitor switchingelement 308. The connection structure of these circuit elements are thesame as that in FIG. 5.

The first phase-inversion element 301 in FIG. 6 has a structure in whicha p-channel transistor 309 and an n-channel transistor 310 whose gateelectrodes are connected to each other are connected in series between afirst node to which a high-level power supply potential VDD is suppliedand a second node to which a low-level power supply potential VSS issupplied. Specifically, a source electrode of the p-channel transistor309 is connected to the first node to which the power supply potentialVDD is supplied, and a source electrode of the n-channel transistor 310is connected to the second node to which the power supply potential VSSis supplied. In addition, a drain electrode of the p-channel transistor309 is connected to a drain electrode of the n-channel transistor 310,and potentials of the two drain electrodes can be regarded as apotential of the output terminal of the first phase-inversion element301. In addition, potentials of the gate electrode of the p-channeltransistor 309 and the gate electrode of the n-channel transistor 310can be regarded as a potential of the input terminal of the firstphase-inversion element 301.

The second phase-inversion element 302 in FIG. 6 has a structure inwhich a p-channel transistor 311 and an n-channel transistor 312 whosegate electrodes are connected to each other are connected in seriesbetween the first node to which the high-level power supply potentialVDD is supplied and the second node to which the low-level power supplypotential VSS is supplied. Specifically, a source electrode of thep-channel transistor 311 is connected to the first node to which thepower supply potential VDD is supplied, and a source electrode of then-channel transistor 312 is connected to the second node to which thepower supply potential VSS is supplied. In addition, a drain electrodeof the p-channel transistor 311 is connected to a drain electrode of then-channel transistor 312, and potentials of the two drain electrodes canbe regarded as a potential of the output terminal of the secondphase-inversion element 302. In addition, potentials of the gateelectrode of the p-channel transistor 311 and the gate electrode of then-channel transistor 312 can be regarded as a potential of the inputterminal of the second phase-inversion element 302.

In FIG. 6, the case where a transistor is used for the switching element303 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 1 supplied to a gate electrode thereof. Inaddition, the case where a transistor is used for the switching element304 is illustrated as an example, and the switching of the transistor iscontrolled by a signal Sig 2 supplied to a gate electrode thereof.

Note that in FIG. 6, a structure in which each of the switching element303 and the switching element 304 includes only one transistor isillustrated; however, the present invention is not limited to thisstructure. In one embodiment of the present invention, the switchingelement 303 or the switching element 304 may include a plurality oftransistors. In the case where the plurality of transistors which serveas switching elements are included in the switching element 303 or theswitching element 304, the plurality of transistors may be connected toeach other in parallel, in series, or in combination of parallelconnection and series connection.

In FIG. 6, a transistor including an oxide semiconductor in a channelformation region is used for the capacitor switching element 306, andthe switching of the transistor is controlled by a signal Sig 3 suppliedto a gate electrode thereof. Since the transistor used for the capacitorswitching element 306 includes a highly-purified oxide semiconductor ina channel formation region, OFF-state current is extremely low asdescribed above.

In FIG. 6, a transistor including an oxide semiconductor in a channelformation region is used for the capacitor switching element 308, andthe switching of the transistor is controlled by a signal Sig 4 suppliedto a gate electrode thereof. Since the transistor used for the capacitorswitching element 308 includes a highly-purified oxide semiconductor ina channel formation region, OFF-state current is extremely low asdescribed above.

Note that in FIG. 6, a structure in which the capacitor switchingelement 306 or the capacitor switching element 308 includes only onetransistor is illustrated; however, the present invention is not limitedto this structure. In one embodiment of the present invention, thecapacitor switching element 306 or the capacitor switching element 308may include a plurality of transistors. In the case where the pluralityof transistors which serve as switching elements are included in thecapacitor switching element 306 or the capacitor switching element 308,the plurality of transistors may be connected to each other in parallel,in series, or in combination of parallel connection and seriesconnection.

In one embodiment of the present invention, at least a transistor usedfor a switching element in the capacitor switching element 306 or thecapacitor switching element 308 may include a highly-purified oxidesemiconductor in a channel formation region. Accordingly, a transistorused for the first phase-inversion element 301, the secondphase-inversion element 302, the switching element 303, or the switchingelement 304 can include a semiconductor other than an oxidesemiconductor, such as amorphous silicon, microcrystalline silicon,polycrystalline silicon, single crystal silicon, amorphous germanium,microcrystalline germanium, polycrystalline germanium, or signal crystalgermanium. In addition, in the above-described transistor, a thinsemiconductor film or a bulk semiconductor substrate may be used. Aslong as a p-channel transistor including an oxide semiconductor film canbe manufactured, all of the transistors in the memory element caninclude the oxide semiconductor film as an active layer, so that theprocess can be simplified.

Next, an example of the operation of the memory element illustrated inFIG. 5 or FIG. 6 will be described.

First, in writing data, the switching element 303 is turned on, theswitching element 304 is turned off, the capacitor switching element 306is turned off, and the capacitor switching element 308 is turned off.Then, the power supply potential VDD is supplied to the first node andthe power supply potential VSS is supplied to the second node, whereby apower supply voltage is applied between the first node and the secondnode. A potential of the signal IN supplied to the memory element 300 issupplied to the input terminal of the first phase-inversion element 301via the switching element 303, whereby the potential of the outputterminal of the first phase-inversion element 301 is a phase-invertedpotential of the signal IN. Then, the switching element 304 is turned onand the input terminal of the first phase-inversion element 301 isconnected to the output terminal of the second phase-inversion element302, whereby data is written into the first phase-inversion element 301and the second phase-inversion element 302.

Next, in the case where the input data is held in the firstphase-inversion element 301 and the second phase-inversion element 302,in the state where the switching element 304 remains in an ON state, thecapacitor switching element 306 remains in an OFF state, and thecapacitor switching element 308 remains in an OFF state, the switchingelement 303 is turned off. By turning off the switching element 303, theinput data is held in the first phase-inversion element 301 and thesecond phase-inversion element 302. At this time, the power supplypotential VDD is supplied to the first node and the power supplypotential VSS is supplied to the second node, whereby the state in whichthe power supply voltage is applied between the first node and thesecond node is maintained.

The potential of the output terminal of the first phase-inversionelement 301 reflects the data held in the first phase-inversion element301 and the second phase-inversion element 302. Therefore, by readingout the potential, the data can be read out from the memory element 300.

Note that in order to reduce power consumption in holding the data, inthe case where the input data is held in the capacitor 305 and thecapacitor 307, the switching element 303 is turned off, the switchingelement 304 is turned on, the capacitor switching element 306 is turnedon, and the capacitor switching element 308 is turned on. Then, via thecapacitor switching element 306, charge with an amount corresponding tothe value of the data held in the first phase-inversion element 301 andthe second phase-inversion element 302 is stored in the capacitor 305,whereby the data is written into the capacitor 305. In addition, via thecapacitor switching element 308, charge with an amount corresponding tothe value of the data held in the first phase-inversion element 301 andthe second phase-inversion element 302 is stored in the capacitor 307,whereby the data is written into the capacitor 307. Note that thepolarity of a voltage between a pair of electrodes included in thecapacitor 305 and a voltage between a pair of electrodes included in thecapacitor 307 are opposite to each other.

After the data is stored in the capacitor 305, the capacitor switchingelement 306 is turned off, whereby the data stored in the capacitor 305is held. In addition, after the data is stored in the capacitor 307, thecapacitor switching element 308 is turned off, whereby the data storedin the capacitor 307 is held. After turning off the capacitor switchingelement 306 and the capacitor switching element 308, for example, thepower supply potential VSS is supplied to each of the first node and thesecond node so that the nodes have equal potentials, and the applicationof the power supply voltage between the first node and the second nodeis stopped.

In such a manner, in the case where the input data is held in thecapacitor 305 and the capacitor 307, the application of the power supplyvoltage between the first node and the second node is unnecessary;therefore, via the p-channel transistor 309 and the n-channel transistor310 which are included in the first phase-inversion element 301, or viathe p-channel transistor 311 and the n-channel transistor 312 which areincluded in the second phase-inversion element 302, the OFF-statecurrent flowing between the first node and the second node can beextremely close to zero. As a result, power consumption due to theOFF-state current of the memory element in holding the data can besignificantly reduced, and the power consumption of the memory deviceand further the whole semiconductor device including the memory devicecan be suppressed to be low.

Since the transistor used for each of the capacitor switching element306 and the capacitor switching element 308 includes a highly-purifiedoxide semiconductor in a channel formation region, OFF-state currentdensity can be less than or equal to 100 zA/μm, preferably less than orequal to 10 zA/μm, more preferably less than or equal to 1 zA/μm.Accordingly, the transistor including the highly-purified oxidesemiconductor film as an active layer has far lower OFF-state currentthan a transistor including silicon having crystallinity. As a result,when the capacitor switching element 306 for which the transistor isused is in an OFF state, charge stored in the capacitor 305 is hardlyreleased; therefore, the data is held. In addition, when the capacitorswitching element 308 for which the transistor is used is in an OFFstate, charge stored in the capacitor 307 is hardly released; therefore,the data is held.

In the case where the data stored in the capacitor 305 and the capacitor307 is read out, the power supply potential VDD is supplied to the firstnode and the power supply potential VSS is supplied to the second node,whereby the power supply voltage is applied between the first node andthe second node. In this state, the capacitor switching element 306 isturned on. When the power supply voltage is applied between the firstnode and the second node, the output terminal of the firstphase-reversing element 301 is supplied with a phase-reversed potentialof the potential of the input terminal thereof. Note that the inputterminal of the first phase-inversion element 301 is supplied with apotential having a level corresponding to the amount of charge stored inthe capacitor 305; thus, a potential of the output terminal thereofreflects the data. In addition, by turning on the capacitor switchingelement 308, a potential having a level corresponding to the amount ofcharge stored in the capacitor 305 is supplied to the output terminal ofthe first phase-inversion element 301. Thus, the signal OUT having apotential that reflects the data can be read out from the memory element300.

FIG. 28 is an example of a timing chart in the case where the supply ofthe power supply potential VDD is not stopped in the circuit of FIG. 6.When the signal Sig 1 is set to a low level and the signal Sig 2 is setto a high level, the signal IN is blocked, and a feedback loop isformed, and the state can be held. When the signal Sig 1 is set to ahigh level again and the signal Sig 2 is a set to a low level, thesignal IN is input and output via the first phase-inversion element 301.At this time, the signal Sig 3 and the signal Sig 4 are set to a lowlevel.

FIG. 29 is an example of a timing chart in the case where the supply ofthe power supply potential VDD is stopped in the circuit of FIG. 6. Whenthe signal Sig 1 is set to a low level and the signal Sig 2 is set to ahigh level, the signal IN is blocked, and a feedback loop is formed, andthe state can be held. After that, the signal Sig 3 and the signal Sig 4are set to a high level, whereby the capacitor 305 and the capacitor 307store the data. Even when the supply of the power supply potential VDDis stopped after that, potentials of the capacitor 305 amd the capacitor307 are held (in FIG. 29, the capacitor 305 has a high-level potential,and the capacitor 307 has a low-level potential). After that, when thepower supply potential VDD is supplied and the signal Sig 3 and thesignal Sig 4 are set to a high level again, the potential of thecapacitor 305 is output via the first phase-inversion element 301, andthe potential of the capacitor 307 is output (in FIG. 29, both thecapacitor 305 and the capacitor 307 have a low-level potential).

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

Embodiment 4

A semiconductor device which is one embodiment of the present inventionincludes a transistor including silicon and a transistor including anoxide semiconductor. The transistor including silicon can be formedusing a silicon wafer, an SOI (silicon on insulator) substrate, a thinsilicon film over an insulating surface, or the like.

An SOI substrate can be manufactured using, for example, UNIBOND(registered trademark) typified by Smart Cut (registered trademark),epitaxial layer transfer (ELTRAN) (registered trademark), a dielectricseparation method, plasma assisted chemical etching (PACE), separationby implanted oxygen (SIMOX), or the like.

A semiconductor film of silicon formed over a substrate having aninsulating surface may be crystallized by a known technique. As theknown technique of crystallization, a laser crystallization method usinga laser beam and a crystallization method using a catalytic element aregiven. Alternatively, a crystallization method using a catalytic elementand a laser crystallization method may be combined. In the case of usinga heat-resistant substrate such as quartz, it is possible to combine anyof the following crystallization methods: a thermal crystallizationmethod with an electrically heated oven, a lamp anneal crystallizationmethod with infrared light, a crystallization method with a catalyticelement, and high temperature annealing method at about 950° C.

In addition, a semiconductor element manufactured using theabove-described method may be transferred onto a flexible substrateformed of plastic or the like, so that a semiconductor device is formed.A variety of transfer methods can be used. Examples of the transfermethods include a method in which a metal oxide film is provided betweenthe substrate and the semiconductor element, and the metal oxide film isembrittled by crystallization so that the semiconductor element isseparated off and transferred; a method in which an amorphous siliconfilm containing hydrogen is provided between the substrate and thesemiconductor element, and the amorphous silicon film is removed bylaser beam irradiation or etching so that the semiconductor element isseparated off from the substrate and transferred; a method in which thesubstrate, for which the semiconductor element is provided, is removedby mechanical cutting or etching by a solution or a gas so that thesemiconductor element is cut off from the substrate and transferred; andthe like.

In this embodiment, a structure of a semiconductor device and a methodfor manufacturing the semiconductor device will be described by givingan example in which with the use of an SOI (silicon on insulator)substrate, the transistor including silicon is manufactured and then thetransistor including an oxide semiconductor is manufactured.

First, as illustrated in FIG. 7A, a bond substrate 500 is cleaned, andthen, an insulating film 501 is formed over a surface of the bondsubstrate 500.

As the bond substrate 500, a single crystal semiconductor substrateformed using silicon can be used. Alternatively, the bond substrate 500may be a semiconductor substrate formed using silicon having crystallattice distortion, silicon germanium obtained by adding germanium tosilicon, or the like.

Note that in a single crystal semiconductor substrate used for the bondsubstrate 500, the directions of crystal axes are preferably uniform;however, the substrate is not necessarily formed using perfect crystalsin which a lattice defect such as a point defect, a line defect, or aplane defect is completely eliminated.

In addition, the shape of the bond substrate 500 is not limited to acircle, and the substrate can be processed into a shape other than acircle. The bond substrate 500 may be processed into, for example, arectangular shape considering that the shape of a base substrate 503 towhich the bond substrate 500 is attached later is generally a rectangleand that a light exposure region of a light exposure apparatus such as areduced projection exposure apparatus is rectangular, and the like. Thebond substrate 500 can be processed by cutting a circular single crystalsemiconductor substrate that is commercially available.

The insulating film 501 may be a single insulating film or a stack ofplural insulating films. Considering that a region containing impuritieswill be removed later, the thickness of the insulating film 501 ispreferably greater than or equal to 15 nm and less than or equal to 500nm.

As a film included in the insulating film 501, an insulating filmcontaining silicon or germanium as its component such as a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, a siliconnitride oxide film, a germanium oxide film, a germanium nitride film, agermanium oxynitride film, or a germanium nitride oxide film can beused. Further, an insulating film containing a metal oxide such asaluminum oxide, tantalum oxide, or hafnium oxide; an insulating filmcontaining a metal nitride such as aluminum nitride; an insulating filmcontaining a metal oxynitride such as an aluminum oxynitride film; or aninsulating film containing a metal nitride oxide such as an aluminumnitride oxide film can be used.

In this embodiment, an example in which silicon oxide formed by thermaloxidation of the bond substrate 500 is used as the insulating film 501is described. Note that in FIG. 7A, the insulating film 501 is formed soas to cover the entire surface of the bond substrate 500; however, theinsulating film 501 may be formed on at least one surface of the bondsubstrate 500.

Note that in this specification, an oxynitride is a substance containingoxygen and nitrogen so that the amount of oxygen is larger than that ofnitrogen, whereas a nitride oxide is a substance containing oxygen andnitrogen so that the amount of nitrogen is larger than that of oxygen.

In the case where the insulating film 501 is formed by thermal oxidationof the surface of the bond substrate 500, dry oxidation in which oxygenhaving a small amount of moisture is used, thermal oxidation in whichgas including a halogen such as hydrogen chloride is added to an oxygenatmosphere, or the like can be used as the thermal oxidation. Inaddition, wet oxidation such as pyrogenic oxidation in which hydrogen isburnt with oxygen to generate water, or water vapor oxidation in whichhigh-purity water is heated at 100° C. or higher to generate water vaporand oxidation is performed with use of the water vapor may be used forforming the insulating film 501.

In the case where the base substrate 503 includes impurities whichdecrease the reliability of a semiconductor device, such as an alkalimetal or an alkaline earth metal, the insulating film 501 preferablyincludes at least one layer of a barrier film that can prevent theimpurities from diffusing from the base substrate 503 into asemiconductor film which is to be formed after separation. As theinsulating film that can be used as the barrier film, a silicon nitridefilm, a silicon nitride oxide film, an aluminum nitride film, analuminum nitride oxide film, or the like can be given. The insulatingfilm that is used as the barrier film is preferably formed to athickness of 15 nm to 300 nm, for example. In addition, an insulatingfilm which has a lower proportion of nitrogen than the barrier film,such as a silicon oxide film or a silicon oxynitride film may be formedbetween the barrier film and the bond substrate 500. The insulating filmwhich has a lower proportion of nitrogen may be formed to a thickness ofgreater than or equal to 5 nm and less than or equal to 200 nm.

In the case of using silicon oxide for the insulating film 501, theinsulating film 501 can be formed by a vapor deposition method such asthermal CVD, plasma CVD, normal pressure CVD, or bias ECRCVD with theuse of a mixed gas of silane and oxygen, a mixed gas of TEOS(tetraethoxysilane) and oxygen, or the like. In this case, a surface ofthe insulating film 501 may be subjected to oxygen plasma treatment toincrease the density. Meanwhile, in the case of using silicon nitridefor the insulating film 501, the insulating film 501 can be formed by avapor deposition method such as plasma CVD with the use of a mixed gasof silane and ammonia.

Alternatively, the insulating film 501 may be formed using silicon oxideby a chemical vapor deposition method with the use of an organosilanegas. As an organosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄),tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula:SiH(OC₂H₅)₃), or trisdimethylaminosilane (chemical formula:SiH(N(CH₃)₂)₃) can be used.

With the use of an organosilane gas for the source gas, a silicon oxidefilm with a flat surface can be formed at a process temperature of 350°C. or lower. Alternatively, the insulating film can be formed by athermal CVD method using a low temperature oxide (LTO) which is formedat a heating temperature of higher than or equal to 200° C. and lowerthan or equal to 500° C. LTO can be formed by using monosilane (SiH₄),disilane (Si₂H₆), or the like as a silicon source gas and using nitrogendioxide (NO₂) or the like as an oxygen source gas.

For example, in the case of using TEOS and O₂ for the source gas to formthe silicon oxide film as the insulating film 501, the condition may beset as follows: the flow rate of TEOS is 15 sccm, the flow rate of O₂ is750 sccm, the deposition pressure is 100 Pa, the deposition temperatureis 300° C., the RF output is 300 W, and the power source frequency is13.56 MHz.

Note that an insulating film formed at a relatively low temperature,such as a silicon oxide film formed using organosilane or a siliconnitride oxide film formed at a low temperature, has a number of OHgroups on its surface. Hydrogen bonding between the OH group and a watermolecule forms a silanol group and bonds the base substrate and theinsulating film at a low temperature. A siloxane bond, which is acovalent bond, is formed finally between the base substrate and theinsulating film. The insulating film such as the above-described siliconoxide film formed using organosilane or the LTO formed at a relativelylow temperature is suitable for bonding at a low temperature, ascompared with a thermally oxidized film having no OH groups or havingvery few OH groups which is used in Smart Cut (registered trademark) orthe like.

The insulating film 501 forms a bonding plane which is flat andhydrophilic over the surface of the bond substrate 500. Therefore, theaverage surface roughness R_(a) of the insulating film 501 is preferablyless than or equal to 0.7 nm, more preferably less than or equal to 0.4nm. The thickness of the insulating film 501 may be greater than orequal to 5 nm and less than or equal to 500 nm, preferably greater thanor equal to 10 nm and less than or equal to 200 nm.

Next, as illustrated in FIG. 7B, the bond substrate 500 is irradiatedwith an ion beam including ions accelerated by an electric field throughthe insulating film 501 as indicated by arrows, whereby an embrittlementlayer 502 having microvoids is formed in a region at a predetermineddepth from the surface of the bond substrate 500. For example, theembrittlement layer means a layer which is locally embrittled bydisorder of a crystal structure, and the state of the embrittlementlayer depends on a means for forming the embrittlement layer. Note thatthere may be a case where a region ranging from one surface of the bondsubstrate to the embrittlement layer is embrittled to some extent;however, the embrittlement layer in this specification indicates aregion at which separation is performed later and its vicinity.

The depth at which the embrittlement layer 502 is formed can be adjustedby the acceleration energy of the ion beam and the angle at which theion beam enters. The embrittlement layer 502 can be formed at the sameor substantially the same depth as the average penetration depth of theions. The thickness of a semiconductor film 504 which will be separatedfrom the bond substrate 500 is determined by the depth at which the ionsare implanted. The depth at which the embrittlement layer 502 is formedcan be set in the range of, for example, greater than or equal to 50 nmand less than or equal to 500 nm, preferably greater than or equal to 50nm and less than or equal to 200 nm.

The ions are implanted to the bond substrate 500 desirably by an iondoping method in which mass separation is not performed because thecycle time can be shortened; however, the present invention may employan ion implantation method in which mass separation is performed.

When hydrogen (H₂) is used for a source gas, H⁺, H₂ ⁺, and H₃ ⁺ can beproduced by exciting a hydrogen gas. Proportions of ion species producedfrom the source gas can be changed by adjusting a plasma excitationmethod, the pressure of an atmosphere for generating plasma, the amountof supplied source gas, or the like. In the case where the ionimplantation is performed by an ion doping method, it is preferable thatH₃ ⁺ be contained at 50% or more, more preferably at 80% or more, withrespect to the total amount of H⁺, H₂ ⁺, and H₃ ⁺ in the ion beam. WhenH₃ ⁺ is contained at 80% or higher, the proportion of H₂ ⁺ ions in theion beam gets lower relatively, which results in smaller variation inthe average penetration depth of the hydrogen ions contained in the ionbeam. Consequently, the ion implantation efficiency improves and thecycle time can be shortened.

H₃ ⁺ has larger mass than H⁺ and H₂ ⁺. When the ion beam containing ahigher proportion of H₃ ⁺ is compared with the ion beam containing ahigher proportion of H⁺ and H₂ ⁺, the former can implant hydrogen into ashallower region of the bond substrate 500 than the latter even when theacceleration voltage at the time of doping is the same. Moreover, theformer has a steep concentration distribution of hydrogen implanted intothe bond substrate 500 in a thickness direction, therefore, thethickness of the embrittlement layer 502 itself can be smaller.

In the case of performing ion implantation by an ion doping method withthe use of a hydrogen gas, the acceleration voltage is set to greaterthan or equal to 10 kV and less than or equal to 200 kV and the dose isset to greater than or equal to 1×10¹⁶ ions/cm² and less than or equalto 6×10¹⁶ ions/cm². Under this condition, the embrittlement layer 502can be formed in a region at a depth of greater than or equal to 50 nmand less than or equal to 500 nm of the bond substrate 500, althoughdepending on the ion species included in the ion beam and itsproportion, and the film thickness of the insulating film 501.

For example, in the case where the bond substrate 500 is a singlecrystal silicon substrate and the insulating film 501 is formed using a100-nm-thick thermal oxide film, a semiconductor film with a thicknessof approximately 146 nm can be separated from the bond substrate 500under the condition where the flow rate of 100% hydrogen gas, which isthe source gas, is 50 sccm, the beam current density is 5 μA/cm², theacceleration voltage is 50 kV, and the dose is 2.0×10¹⁶ atoms/cm². Notethat even when the condition at the time of adding hydrogen to the bondsubstrate 500 is not changed, the thickness of the insulating film 501is made larger, whereby the thickness of the semiconductor film can bemade smaller.

Helium (He) can alternatively be used as the source gas of the ion beam.Since most of the ion species produced by exciting helium are He⁺, He⁺can be mainly implanted into the bond substrate 500 even by an iondoping method in which mass separation is not performed. Therefore,microvoids can be formed in the embrittlement layer 502 efficiently byan ion-doping method. In the case of performing ion implantation by anion doping method using helium, the acceleration voltage can be greaterthan or equal to 10 kV and less than or equal to 200 kV, and the dosecan be greater than or equal to 1×10¹⁶ ions/cm² and less than or equalto 6×10¹⁶ ions/cm².

A halogen gas such as a chlorine gas (Cl₂ gas) or a fluorine gas (F₂gas) can be used as a source gas.

In the case of performing ion implantation on the bond substrate 500 byan ion doping method, impurities existing in an ion doping apparatus areimplanted together with the ions to a processing object; therefore,there is a possibility that impurities such as S, Ca, Fe, and Mo existon and near the surface of the insulating film 501. Therefore, a regionon and near the surface of the insulating film 501 where the number ofimpurities is considered to be the largest may be removed by etching,polishing, or the like. Specifically, a region at a depth of 10 nm to100 nm, preferably approximately 30 nm to 70 nm from the surface of theinsulating film 501 may be removed. The etching may be performed by adry etching method such as a reactive ion etching (RIE) method; forexample, an inductively coupled plasma (ICP) etching method, an electroncyclotron resonance (ECR) etching method, a parallel plate (capacitivecoupled type) etching method, a magnetron plasma etching method, adual-frequency plasma etching method, a helicon wave plasma etchingmethod, or the like can be used. For example, in the case of removing aregion on and near a surface of a silicon nitride oxide film by an ICPetching method, the region can be removed to a depth of about 50 nm fromthe surface under the condition where the flow rate of CHF₃ as anetching gas is 7.5 sccm, the flow rate of He is 100 sccm, the reactionpressure is 5.5 Pa, the temperature of a lower electrode is 70° C., theRF (13.56 MHz) electric power applied to a coil-shaped electrode is 475W, the electric power applied to the lower electrode (on bias side) is300 W, and the etching time is about 10 seconds.

Instead of CHF₃, which is a fluorine-based gas, a chlorine-based gassuch as Cl₂, BCl₃, SiCl₄, or CCl₄; another fluorine-based gas such asCF₄, SF₆, or NF₃; or O₂ can be used as appropriate for the etching gas.Moreover, an inert gas other than He may be added to the etching gas.For example, one or plural elements selected from Ne, Ar, Kr, and Xe canbe used as the inert element which is added to the etching gas. In thecase of removing a region on and near a surface of a silicon nitrideoxide film by wet etching, an aqueous solution containing ammoniumhydrogen fluoride, ammonium fluoride, or the like, may be used as anetchant. The polishing can be performed by CMP (chemical mechanicalpolishing), liquid jet polishing, or the like.

After the formation of the embrittlement layer 502, an extremelycontaminated region on and near the surface of the insulating film 501is removed by etching, polishing, or the like, whereby the amount ofimpurities which enter the semiconductor film 504 formed over the basesubstrate 503 can be suppressed. Moreover, in a semiconductor devicewhich is completed finally, it is possible to prevent the impuritiesfrom causing decrease in reliability and decrease in electriccharacteristics of transistors, such as variation in threshold voltageor increase in leakage current.

In order to remove the impurities, it is possible to perform atomic beamor ion beam irradiation treatment, plasma treatment, or radicaltreatment. In the case of using an atomic beam or an ion beam, an inertgas neutral atom beam or an inert gas ion beam of argon or the like canbe used.

Next, as illustrated in FIG. 7C, the bond substrate 500 and the basesubstrate 503 are attached to each other so that the insulating film 501is interposed therebetween.

Note that before the attachment of the base substrate 503 and the bondsubstrate 500, surfaces for the attachment, i.e., a surface of theinsulating film 501 formed over the bond substrate 500 and a surface ofthe base substrate 503 in this embodiment, are preferably subjected tosurface treatment for increasing bonding strength between the insulatingfilm 501 and the base substrate 503.

As examples of the surface treatment, wet treatment, dry treatment, andcombination of wet treatment and dry treatment can be given. Differentwet treatment or different dry treatment may be combined to beperformed. Examples of the wet treatment include ozone treatment usingozone water (ozone water cleaning), ultrasonic cleaning such asmegasonic cleaning, two-fluid cleaning (a method in which functionalwater such as pure water or hydrogenated water and a carrier gas such asnitrogen are sprayed together), cleaning with hydrochloric acid and ahydrogen peroxide solution, and the like. As examples of the drytreatment, inert gas neutral atomic beam treatment, inert gas ion beamtreatment, ultraviolet treatment, ozone treatment, plasma treatment,plasma treatment with bias application, radical treatment, and the likecan be given. By performing the above-described surface treatment, thehydrophilicity and cleanliness of the surfaces for the attachment can beincreased. Thus, the bonding strength can be improved.

For the attachment, the base substrate 503 and the insulating film 501formed over the bond substrate 500 are disposed in close contact witheach other, and then, a pressure of approximately greater than or equalto 1 N/cm² and less than or equal to 500 N/cm², preferably greater thanor equal to 11 N/cm² and less than or equal to 20 N/cm² is applied topart of the base substrate 503 and the bond substrate 500 which aresuperposed on each other. When the pressure is applied, bonding betweenthe base substrate 503 and the insulating film 501 starts from theportion, which results in bonding of the entire surface where the basesubstrate 503 and the insulating film 501 are in close contact with eachother.

The bonding is performed by van der Waals force or a hydrogen bonding,so that the bonding is firm even at room temperature. Note that sincethe above-described bonding can be performed at a low temperature, avariety of substrates can be used for the base substrate 503. Forexample, a variety of glass substrates for electronics industry, such asan alumino silicate glass substrate, a barium borosilicate glasssubstrate, or an aluminoborosilicate glass substrate, a quartzsubstrate, a ceramic substrate, a sapphire substrate, or the like can beused as the base substrate 503. Alternatively, as the base substrate503, a semiconductor substrate formed of silicon, gallium arsenide,indium phosphide, or the like can be used. Further alternatively, thebase substrate 503 may be a metal substrate including a stainless steelsubstrate. Substrates with coefficients of thermal expansion of greaterthan or equal to 25×10⁻⁷/° C. and less than or equal to 50×10⁻⁷/° C.(preferably, greater than or equal to 30×10⁻⁷/° C. and less than orequal to 40×10⁻⁷/° C.) and strain points of greater than or equal to580° C. and less than or equal to 680° C. (preferably, greater than orequal to 600° C. and less than or equal to 680° C.) are preferably usedas the glass substrate which serves as the base substrate 503. When theglass substrate is an alkali-free glass substrate, impuritycontamination of semiconductor devices can be suppressed.

As the glass substrate, a mother glass substrate developed forproduction of liquid crystal panels can be used. As such a mother glasssubstrate, substrates having the following sizes are known: the thirdgeneration (550 mm×650 mm), the 3.5-th generation (600 mm×720 mm), thefourth generation (680 mm×880 mm, or 730 mm×920 mm), the fifthgeneration (1100 mm×1300 mm), the sixth generation (1500 mm×1850 mm),the seventh generation (1870 mm×2200 mm), the eighth generation (2200mm×2400 mm), and the like. When an SOI substrate is manufactured with alarge-area mother glass substrate used as the base substrate 503, alarge-area SOI substrate can be obtained. Size increase of an SOIsubstrate can be realized by using a large substrate such as a motherglass substrate, as the base substrate 503. When the SOI substrate canhave a larger area, a large number of chips such as ICs or LSIs can bemanufactured by using one substrate. Thus, productivity can bedramatically increased.

Although there is no particular limitation on a substrate which can beused as the base substrate 503, it is necessary that the substrate haveat least enough heat resistance to heat treatment performed later. Forexample, a glass substrate manufactured by a fusion method or a floatmethod can be used. When the temperature of the heat treatment performedlater is high, a substrate having a strain point of 730° C. or higher ispreferably used as the glass substrate. As a glass substrate, a glassmaterial such as aluminosilicate glass, aluminoborosilicate glass, orbarium borosilicate glass is used, for example. In general, bycontaining barium oxide (BaO) and boron oxide so that the amount ofbarium oxide is larger than that of boron oxide, a glass substrate thatis heat-resistant and of more practical use can be obtained. Therefore,a glass substrate containing BaO and B₂O₃ so that the amount of BaO islarger than that of B₂O₃ is preferably used. Note that in order to avoidsuch defective bonding that is caused by the shrink, the base substrate503 may be subjected to heat treatment in advance before the bondingstep.

Moreover, an insulating film may be formed in advance over the basesubstrate 503. The base substrate 503 is not necessarily provided withan insulating film on its surface. However, the formation of theinsulating film on the surface of the base substrate 503 can preventimpurities of the base substrate 503, such as an alkali metal and analkaline earth metal, from entering the bond substrate 500. Moreover, inthe case of forming the insulating film on the surface of the basesubstrate 503, the insulating film over the base substrate 503 is bondedto the insulating film 501; therefore, a wider variety of substrates canbe used as the base substrate 503. In general, the upper temperaturelimits of substrates formed of flexible synthetic resins such asplastics tend to be low. However, as long as the substrates can resistprocess temperatures in manufacturing steps of a semiconductor elementperformed later, the substrates formed of such resins can be used as thebase substrate 503 in the case of forming the insulating film over thebase substrate 503. Examples of a plastic substrate include polyestertypified by polyethylene terephthalate (PET), polyethersulfone (PES),polyethylene naphthalate (PEN), polycarbonate (PC), polyetheretherketone(PEEK), polysulfone (PSF), polyetherimide (PEI), polyarylate (PAR),polybutylene terephthalate (PBT), polyimide,acrylonitrile-butadiene-styrene resin, polyvinyl chloride,polypropylene, polyvinyl acetate, acrylic resin, and the like. In thecase of forming the insulating film over the base substrate 503, theattachment of the base substrate 503 and the bond substrate 500 to eachother is preferably performed after surface treatment is performed onthe surface of this insulating film in a manner similar to that for theinsulating film 501.

After the bond substrate 500 is attached to the base substrate 503, heattreatment is preferably performed in order to increase the bondingstrength at the bonding interface between the base substrate 503 and theinsulating film 501. This treatment is performed at a temperature wherea crack is not generated in the embrittlement layer 502 and can beperformed at a temperature in the range of higher than or equal to 200°C. and lower than or equal to 400° C. By attaching the bond substrate500 to the base substrate 503 within this temperature range, the bondingstrength between the base substrate 503 and the insulating film 501 canbe made firm.

When the bonding plane is contaminated by dust or the like at the timeof attaching the bond substrate 500 and the base substrate 503 to eachother, the contaminated portion is not bonded. In order to avoid thecontamination of the bonding plane, the attachment of the bond substrate500 and the base substrate 503 is preferably performed in an airtightchamber. At the time of attaching the bond substrate 500 and the basesubstrate 503 to each other, the treatment chamber may have pressurereduced to approximately 5.0×10⁻³ Pa and the atmosphere of the bondingtreatment may be cleaned.

Subsequently, heat treatment is performed whereby microvoids adjacent toeach other in the embrittlement layer 502 are combined, so that themicrovoids increase in volume. As a result, as illustrated in FIG. 7D,the semiconductor film 504 which is part of the bond substrate 500 isseparated from the bond substrate 500 along the embrittlement layer 502.Since the insulating film 501 and the base substrate 503 are bonded toeach other, the semiconductor film 504 which is separated from the bondsubstrate 500 is fixed to the base substrate 503. The heat treatment forseparating the semiconductor film 504 from the bond substrate 500 ispreferably performed at a temperature which does not exceed the strainpoint of the base substrate 503.

For this heat treatment, a rapid thermal annealing (RTA) apparatus, aresistive heating furnace, or a microwave heating apparatus can be used.For the RTA apparatus, a gas rapid thermal annealing (GRTA) apparatus ora lamp rapid thermal annealing (LRTA) apparatus can be used. When a GRTAapparatus is used, the heating temperature can be set at a temperatureof higher than or equal to 550° C. and lower than or equal to 650° C.,and the treatment time can be set at greater than or equal to 0.5 minuteand less than or equal to 60 minutes. When a resistance heatingapparatus is used, the heat temperature can be set at higher than orequal to 200° C. and lower than or equal to 650° C. and the treatmenttime can be set at greater than or equal to 2 hours and less than orequal to 4 hours.

The heat treatment may be performed by dielectric heating with ahigh-frequency wave such as a microwave. The heat treatment by thedielectric heating can be performed by irradiating the bond substrate500 with high frequency waves generated at a high-frequency generator,which is in the range of 300 MHz to 3 THz. Specifically, for example,irradiation with a microwave with a frequency of 2.45 GHz at 900 W isperformed for 14 minutes to combine microvoids adjacent to each other inthe embrittlement layer, whereby the bond substrate 500 can be splitalong the embrittlement layer finally.

A specific treatment method of heat treatment using a vertical furnacehaving resistive heating is described. The base substrate 503 to whichthe bond substrate 500 is attached is disposed on a boat of the verticalfurnace and this boat is delivered in a chamber of the vertical furnace.In order to suppress oxidation of the bond substrate 500, the chamber isevacuated first such that a vacuum state is formed. The degree of vacuumis approximately 5×10⁻³ Pa. After a vacuum state is obtained, nitrogenis supplied to the chamber so that the chamber has a nitrogen atmosphereunder atmospheric pressure. In this period, the heating temperature isincreased to 200° C.

After making the chamber have a nitrogen atmosphere that is underatmospheric pressure, heating at 200° C. is performed for 2 hours. Then,the temperature is increased to 400° C. in 1 hour. After the state at aheating temperature of 400° C. becomes stable, the temperature isincreased to 600° C. in 1 hour. After the state at a heating temperatureof 600° C. becomes stable, heat at 600° C. is performed for 2 hours.Then, the heating temperature is decreased to 400° C. in 1 hour, andafter 10 minutes to 30 minutes, the boat is carried out from thechamber. The base substrate 503 to which the bond substrate 500 and thesemiconductor film 504 are attached and which is disposed on the boat iscooled in an air atmosphere.

The heat treatment using the above resistance heating furnace isperformed by successively performing heat treatment for increasing thebonding strength between the insulating film 501 and the base substrate503 and heat treatment for splitting the embrittlement layer 502. In thecase of performing these two kinds of heat treatment in differentapparatuses, for example, heat treatment is performed at 200° C. for 2hours in a resistance heating furnace and then the base substrate 503and the bond substrate 500 which are attached to each other are carriedout from the furnace. Next, heat treatment is performed by an RTAapparatus at a process temperature of higher than or equal to 600° C.and lower than or equal to 700° C. for one minute to several hours, sothat the bond substrate 500 is split along the embrittlement layer 502.

Note that in some cases, a periphery of the bond substrate 500 is notbonded to the base substrate 503. This is seemingly because theperiphery of the bond substrate 500 is chamfered or has a curvature, sothat the base substrate 503 and the insulating film 501 are not in closecontact with each other or the embrittlement layer 502 is difficult tosplit at the periphery of the bond substrate 500. Another reason is thatpolishing such as CMP performed in manufacturing the bond substrate 500is insufficient at the periphery of the bond substrate 500, so that asurface thereof is rougher at the periphery than at a center. Stillanother reason is that, in the case where a carrier or the like damagesthe periphery of the bond substrate 500 at the time of delivery of thebond substrate 500, the damage makes it difficult to bond the peripheryto the base substrate 503. For these reasons, the semiconductor film 504which is smaller than the bond substrate 500 is attached to the basesubstrate 503.

Note that the bond substrate 500 may be subjected to hydrogenationtreatment before the bond substrate 500 is split. Hydrogenationtreatment is performed, for example, at 350° C. for about 2 hours in ahydrogen atmosphere.

When a plurality of bond substrates 500 is attached to the basesubstrate 503, the plural bond substrates 500 may have different crystalplane orientation. The mobility of majority carriers in a semiconductordepends on crystal plane orientation. Therefore, the semiconductor film504 may be formed by selecting as appropriate the bond substrate 500which has crystal plane orientation suitable for a semiconductor elementto be formed. For example, in a case of forming an n-type semiconductorelement by using the semiconductor film 504, the mobility of majoritycarriers in the semiconductor element can be increased by forming thesemiconductor film 504 which has a {100} plane. In contrast, forexample, in the case of forming a p-type semiconductor element by usingthe semiconductor film 504, the mobility of majority carriers in thesemiconductor element can be increased by forming the semiconductor film504 which has a {110} plane. Then, in the case of forming a transistoras a semiconductor element, the bonding direction of the semiconductorfilm 504 is determined in consideration of a channel direction andcrystal plane orientation.

Next, a surface of the semiconductor film 504 may be planarized bypolishing. The planarization is not always necessary; however, theplanarization makes it possible to improve the characteristics of aninterface between semiconductor films 506 and 507 and a gate insulatingfilm which are formed later. Specifically, the polishing may be chemicalmechanical polishing (CMP), liquid jet polishing, or the like. Thethickness of the semiconductor film 504 is reduced by theabove-described planarization. The planarization may be performed on thesemiconductor film 504 before being etched; alternatively, theplanarization may be performed on the semiconductor films 506 and 507formed by etching.

Not the polishing but etching may be performed on the surface of thesemiconductor film 504 in order to planarize the surface of thesemiconductor film 504. The etching may be performed by a dry etchingmethod such as a reactive ion etching (RIE) method; for example, aninductively coupled plasma (ICP) etching method, an electron cyclotronresonance (ECR) etching method, a parallel plate (capacitive coupledtype) etching method, a magnetron plasma etching method, adual-frequency plasma etching method, a helicon wave plasma etchingmethod, or the like may be used.

For example, when an ICP etching method is used, etching may beperformed under the condition where the flow rate of chlorine as anetching gas is 40 sccm to 100 sccm, the electric power applied to a coiltype electrode is 100 W to 200 W, the electric power applied to a lowerelectrode (on the bias side) is 40 W to 100 W, and the reaction pressureis 0.5 Pa to 1.0 Pa. For example, the thickness of the semiconductorfilm 504 can be reduced to about 50 nm to 60 nm by performing theetching under the condition where the flow rate of chlorine as anetching gas is 100 sccm, the reaction pressure is 1.0 Pa, thetemperature of the lower electrode is 70° C., the RF (13.56 MHz)electric power applied to the coil-shaped electrode is 150 W, theelectric power applied to the lower electrode (on the bias side) is 40W, and the etching time is about 25 seconds to 27 seconds. For theetching gas, a chlorine-based gas such as chlorine, boron chloride,silicon chloride, or carbon tetrachloride; a fluorine-based gas such ascarbon tetrafluoride, sulfur fluoride, or nitrogen fluoride; or oxygencan be used as appropriate.

The etching can not only make the semiconductor film 504 to have a filmthickness optimum for a semiconductor element that is to be formed laterbut also planarize the surface of the semiconductor film 504.

Note that in the semiconductor film 504 closely in contact with the basesubstrate 503, crystal defects are formed owing to the formation of theembrittlement layer 502 and the split along the embrittlement layer 502,or planarity of the surface of the semiconductor film 504 is impaired.Thus, in one embodiment of the present invention, in order to reducecrystal defects and improve planarity, the semiconductor film 504 isirradiated with a laser beam after a process of removing an oxide filmsuch as a natural oxide film which is formed on the surface of thesemiconductor film 504.

In this embodiment of the present invention, the semiconductor film 504is immersed in DHF having a hydrogen fluoride concentration of 0.5 wt %for 110 seconds, whereby the oxide film is removed.

The laser beam irradiation is preferably performed with such an energydensity that the semiconductor film 504 is partly melted. This isbecause when the semiconductor film 504 is completely melted, generationof microcrystals due to recrystallization of the semiconductor film 504is accompanied with disordered nucleation of the semiconductor film 504in a liquid phase and crystallinity of the semiconductor film 504 islowered. By partly melting the semiconductor film 504, so-calledlongitudinal growth in which crystal growth proceeds from an unmeltedsolid portion occurs in the semiconductor film 504. Due to therecrystallization by the longitudinal growth, crystal defects of thesemiconductor film 504 are decreased and crystallinity thereof isrecovered. The state in which the semiconductor film 504 is completelymelted indicates the state in which the semiconductor film 504 is meltedto be in a liquid phase to the interface with the insulating film 501.On the other hand, the state in which the semiconductor film 504 ispartly melted indicates the state in which an upper part thereof ismelted and is in a liquid phase and a lower part thereof is in a solidphase.

As this laser beam irradiation, pulsed laser beam irradiation ispreferable for partly melting the semiconductor film 504. For example,in the case of a pulsed laser, the repetition rate is less than or equalto 1 MHz and the pulse width is greater than or equal to 10 nanosecondsand less than or equal to 500 nanoseconds. For example, a XeCl excimerlaser having a repetition rate of 10 Hz to 300 Hz, a pulse width of 25nanoseconds, and a wavelength of 308 nm can be used.

As the laser beam, a fundamental wave or a second harmonic of asolid-state laser, which is selectively absorbed by a semiconductor, ispreferably used. Specifically, for example, a laser beam having awavelength in the range of greater than or equal to 250 nm and less thanor equal to 700 nm can be used. The energy of the laser beam can bedetermined in consideration of the wavelength of the laser beam, theskin depth of the laser beam, the thickness of the semiconductor film504, or the like. For example, in the case where the thickness of thesemiconductor film 504 is approximately 120 nm and a pulsed laser thatemits laser beam having a wavelength of 308 nm is used, the energydensity of the laser beam may be set to 600 mJ/cm² to 700 mJ/cm².

The following pulsed lasers can be used: an Ar laser, a Kr laser, anexcimer laser, a CO₂ laser, a YAG laser, a Y₂O₃ laser, a YVO₄ laser, aYLF laser, a YAlO₃ laser, a glass laser, a ruby laser, an alexandritelaser, a Ti:sapphire laser, a copper vapor laser, and a gold vaporlaser.

In this embodiment, in the case where the thickness of the semiconductorfilm 504 is approximately 146 nm, the laser beam irradiation can beperformed in the following manner. As a laser emitting a laser beam, aXeCl excimer laser (wavelength: 308 nm, pulse width: 20 nanoseconds, andrepetition rate: 30 Hz) is used. The cross section of the laser light isshaped into a linear form with a size of 0.4 mm×120 mm through anoptical system. The semiconductor film 504 is irradiated with the laserbeam with laser scanning speed of 0.5 mm/s. Then, through the laser beamirradiation, a semiconductor film 505 whose crystal defects have beenrepaired is formed as illustrated in FIG. 7E.

Note that the laser beam irradiation is preferably performed in an inertatmosphere such as a rare gas atmosphere or a nitrogen atmosphere, or areduced-pressure atmosphere. In the case of the above atmosphere, thelaser light irradiation may be performed in an airtight chamber whoseatmosphere is controlled. When the chamber is not used, the laser beamirradiation in an inert atmosphere can be achieved by spraying an inertgas such as a nitrogen gas to the surface to be irradiated with thelaser beam. The laser beam irradiation is performed in an inertatmosphere or a reduced-pressure atmosphere instead of an airatmosphere, whereby the natural oxide film is further prevented frombeing formed, cracks or pitch stripes can be prevented from being formedin the semiconductor film 505 which is formed after the laser beamirradiation, planarity of the semiconductor film 505 can be improved,and the applicable energy range for the laser beam can be widened.

The laser beam preferably has its cross section shaped into a linearform with homogenous energy distribution through an optical system.Accordingly, the laser beam irradiation can be performed homogenously athigh throughput. With the beam length of the laser beam longer than oneside of the base substrate 503, the entire semiconductor film 504attached to the base substrate 503 can be irradiated with the laser beamby scanning the laser beam once. When the beam length of the laser beamis shorter than one side of the base substrate 503, the beam length maybe set so that the entire semiconductor film 504 attached to the basesubstrate 503 can be irradiated with the laser light by scanning severaltimes.

In order to perform the laser beam irradiation in a reduced-pressureatmosphere or an inert atmosphere such as a rare gas atmosphere or anitrogen atmosphere, the laser beam irradiation may be performed in anairtight chamber whose atmosphere is controlled. When the chamber is notused, the laser beam irradiation in an inert atmosphere can be achievedby spraying an inert gas such as a nitrogen gas to the surface to beirradiated with the laser beam. The laser beam irradiation is performedin an inert atmosphere or a reduced-pressure atmosphere instead of anair atmosphere, whereby the natural oxide film is further prevented frombeing formed, cracks or pitch stripes can be prevented from being formedin the semiconductor film 505 which is formed after the laser beamirradiation, planarity of the semiconductor film 505 can be improved,and the applicable energy range for the laser beam can be widened.

In the case where the surface of the semiconductor film 504 isplanarized by dry etching before the laser beam irradiation, damagessuch as crystal defects might be generated on and near the surface ofthe semiconductor film 504 owing to the dry etching. However, theabove-described laser beam irradiation can recover even the damagescaused by the dry etching.

Next, after the laser beam irradiation, the surface of the semiconductorfilm 505 may be etched. When the surface of the semiconductor film 505is etched after the laser beam irradiation, the surface of thesemiconductor film 504 is not necessarily etched before the laser beamirradiation. Moreover, when the surface of the semiconductor film 504 isetched before the laser beam irradiation, the surface of thesemiconductor film 505 is not necessarily etched after the laser beamirradiation. Alternatively, the surface of the semiconductor film 505may be etched after the laser beam irradiation and before the laser beamirradiation.

The etching can not only make the semiconductor film 505 to have a filmthickness optimum for a semiconductor element that is to be formed laterbut also planarize the surface of the semiconductor film 505.

After the laser beam irradiation, the semiconductor film 505 ispreferably subjected to heat treatment at higher than or equal to 500°C. and lower than or equal to 650° C. This heat treatment can eliminatedefects of the semiconductor film 505 which have not been repaired bythe laser beam irradiation and can reduce distortion of thesemiconductor film 505. For this heat treatment, a rapid thermalannealing (RTA) apparatus, a resistive heating furnace, or a microwaveheating apparatus can be used. For the RTA apparatus, a gas rapidthermal annealing (GRTA) apparatus or a lamp rapid thermal annealing(LRTA) apparatus can be used. For example, when a resistance heatingfurnace is used, heat treatment may be performed at 600° C. for 4 hours.

Next, as illustrated in FIG. 8A, the semiconductor film 505 is partlyetched to form the island-shaped semiconductor films 506 and 507. Whenthe semiconductor film 505 is further etched, a region of an end portionof the semiconductor film 505 where the bonding strength is notsufficient can be removed. Although the semiconductor films 506 and 507are formed by etching one semiconductor film 505 in this embodiment, thenumber of semiconductor films which are formed is not limited to two.

Note that the surface of the bond substrate 500 from which thesemiconductor film 505 is separated is planarized, whereby asemiconductor film 505 can be separated again from the bond substrate500.

Specifically, the insulating film 501 which remains mainly at edgeportions of the bond substrate 500 is removed by etching or the like. Inthe case where the insulating film 501 is formed using silicon oxide,silicon oxynitride, or silicon nitride oxide, wet etching usinghydrofluoric acid can be employed.

Next, projections formed at the edge portions of the bond substrate 500owing to the separation of the semiconductor film 505 and the remainingembrittlement layer which contains hydrogen excessively are removed. Forthe etching of the bond substrate 500, wet etching is preferably used,and a tetramethylammonium hydroxide (abbreviation: TMAH) solution can beused as an etchant.

Then, the surface of the bond substrate 500 is polished. For thepolishing, CMP can be used. To smooth the surface of the bond substrate500, the surface is desirably polished by approximately 1 μm to 10 μm inthickness. After the polishing, RCA cleaning using hydrofluoric acid orthe like is performed because abrasive particles and the like are lefton the surface of the bond substrate 500.

By reusing the bond substrate 500, the cost of a material of thesemiconductor substrate can be reduced.

In order to control threshold voltage, an impurity element impartingp-type conductivity such as boron, aluminum, or gallium or an impurityelement imparting n-type conductivity such as phosphorus or arsenic maybe added to the semiconductor films 506 and 507. The impurity elementfor controlling threshold voltage may be added to the semiconductor filmbefore being patterned or on the semiconductor films 506 and 507 formedafter the patterning. Alternatively, the impurity element forcontrolling the threshold voltage may be added to a bond substrate.Further alternatively, the impurity element may be added to the bondsubstrate in order to roughly control the threshold voltage, and theimpurity element may be further added to the semiconductor film beforebeing patterned or the semiconductor films 506 and 507 which are formedafter the patterning in order to finely control the threshold voltage.

Next, gate insulating films 508 are formed to cover the semiconductorfilms 506 and 507, as illustrated in FIG. 8B. The gate insulating films508 can be formed by oxidation or nitridation of surfaces of thesemiconductor films 506 and 507 by high-density plasma treatment. Thehigh-density plasma treatment is performed, for example, by using amixed gas of an inert gas such as He, Ar, Kr, or Xe, and oxygen,nitrogen oxide, ammonia, nitrogen, hydrogen, or the like. In this case,by performing excitation of plasma with introduction of a microwave,plasma with a low electron temperature and high density can begenerated. The surfaces of the semiconductor films are oxidized ornitrided by oxygen radicals (which include OH radicals in some cases) ornitrogen radicals (which include NH radicals in some cases) produced bysuch high-density plasma, whereby an insulating film of 1 nm to 20 nmthick, desirably 5 nm to 10 nm thick, is formed in contact with thesemiconductor films. This insulating film of 5 nm to 10 nm thick is usedas the gate insulating film 508. For example, dinitrogen monoxide (N₂O)is diluted with Ar by 1 to 3 times (flow rate) and a microwave (2.45GHz) electric power of 3 kW to 5 kW is applied with a pressure of 10 Pato 30 Pa to oxidize or nitride the surfaces of the semiconductor films506 and 507. By this treatment, an insulating film having a thickness of1 nm to 10 nm (preferably 2 nm to 6 nm) is formed. Further, nitrousoxide (N₂O) and silane (SiH₄) are introduced and a microwave (2.45 GHz)electric power of 3 kW to 5 kW is applied to the insulating film at apressure of 10 Pa to 30 Pa to form a silicon oxynitride film by avapor-phase growth method, which is to be the gate insulating film. Witha combination of a solid-phase reaction and a reaction by a vapordeposition method, the gate insulating film with low interface statedensity and excellent withstand voltage can be formed.

Since the oxidation or nitridation of the semiconductor films by thehigh-density plasma treatment proceeds by a solid-phase reaction, theinterface state density between the gate insulating film 508 and each ofthe semiconductor films 506 and 507 can be drastically decreased.Further, since the semiconductor films 506 and 507 are directly oxidizedor nitrided by the high-density plasma treatment, variation in thicknessof the insulating film to be formed can be suppressed. Moreover, in thecase where the semiconductor film has crystallinity, the surface of thesemiconductor film is oxidized with solid reaction by the high-densityplasma treatment to restrain fast oxidation only in a crystal grainboundary; therefore, the gate insulating film with uniformity and lowinterface state density can be formed. A transistor, in which theinsulating film formed by the high-density plasma treatment is includedin a part of or the entire gate insulating film, may reduce variationsin characteristics.

Alternatively, the gate insulating film 508 may be formed by thermallyoxidizing the semiconductor films 506 and 507. Further alternatively,the gate insulating film 508 may be formed by a plasma CVD method, asputtering method, or the like as a single layer or a stack of layers ofa film containing one or more of silicon oxide, silicon nitride oxide,silicon oxynitride, silicon nitride, hafnium oxide, aluminum oxide, andtantalum oxide.

Then, after forming a conductive film over the gate insulating films 508as illustrated in FIG. 8C, the conductive film is processed (patterned)into a predetermined shape, so that an electrode 509 is formed over eachof the semiconductor films 506 and 507. A CVD method, a sputteringmethod, or the like can be used for forming the conductive film. As theconductive film, tantalum (Ta), tungsten (W), titanium (Ti), molybdenum(Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), or thelike can be used. Moreover, an alloy containing the above-mentionedmetal as the main component or a compound containing the above-mentionedmetal may be used. Alternatively, the conductive film may be formed of asemiconductor such as polycrystalline silicon doped with an impurityelement such as phosphorus which imparts conductivity to thesemiconductor film.

In the case of forming a two-layer conductive film, a first layer can beformed of tantalum nitride or tantalum and a second layer can be formedof tungsten. Moreover, the following combinations are given: tungstennitride and tungsten, molybdenum nitride and molybdenum, aluminum andtantalum, aluminum and titanium, and the like. Since tungsten andtantalum nitride have high heat resistance, heat treatment for thermalactivation can be performed after the two-layer conductive film isformed. Alternatively, as the combination of the two-layer conductivefilm, silicon doped with an impurity element imparting n-typeconductivity and nickel silicide, silicon doped with an impurity elementimparting n-type conductivity and tungsten silicide, or the like can beused.

Note that, although the electrode 509 is formed of a single-layerconductive film in this embodiment, this embodiment is not limited tothis structure. The electrode 509 may be formed of a plurality ofconductive films which is stacked. In the case of using a three-layerstructure which is stacked with more than three conductive films, astacked structure of a molybdenum film, an aluminum film, and amolybdenum film is preferable.

Note that the electrode 509 may be selectively formed by a dropletdischarge method without using a mask.

Note that a droplet-discharge method means a method in which apredetermined pattern is formed by discharging or ejecting dropletscontaining a predetermined composition from fine pores, and an inkjetmethod is included in the category.

After the conductive film is formed, the electrode 509 can be etchedinto a desired tapered shape by using an ICP (inductively coupledplasma) etching method and appropriately controlling the etchingcondition (e.g., the amount of electric power applied to a coiledelectrode layer, the amount of electric power applied to an electrodelayer on the substrate side, or the electrode temperature on thesubstrate side). In addition, angles and the like of the tapered shapesmay also be controlled by a shape of a mask. Note that as an etchinggas, a chlorine-based gas such as chlorine, boron chloride, siliconchloride, or carbon tetrachloride; a fluorine-based gas such as carbontetrafluoride, sulfur fluoride, or nitrogen fluoride; or oxygen can beused as appropriate.

Next, as shown in FIG. 8D, impurity elements imparting one conductivitytype is added to the semiconductor films 506 and 507 by using theelectrodes 509 as masks In this embodiment, an impurity elementimparting n-type conductivity (e.g., phosphorus or arsenic) is added tothe semiconductor film 506, and an impurity element imparting p-typeconductivity (e.g., boron) is added to the semiconductor film 507. Notethat when the impurity element imparting p-type conductivity is added tothe semiconductor film 507, the semiconductor film 506 to which theimpurity element imparting n-type conductivity is to be added is coveredwith a mask or the like so that the impurity element imparting p-typeconductivity is added selectively. On the other hand, when the impurityelement imparting n-type conductivity is added to the semiconductor film506, the semiconductor film 507 to which the impurity element impartingp-type conductivity is to be added is covered with a mask or the like sothat the impurity element imparting n-type conductivity is addedselectively. Alternatively, after adding an impurity element impartingone of p-type and n-type conductivity to the semiconductor films 506 and507, an impurity element imparting the other conductivity may be addedto only one of the semiconductor films 506 and 507 selectively at higherconcentration than the previously added impurity. By the addition of theimpurity elements, impurity regions 510 are formed in the semiconductorfilm 506, and impurity regions 511 are formed in the semiconductor film507.

Next, as illustrated in FIG. 9A, sidewalls 512 are formed on sidesurfaces of the electrodes 509. For example, the sidewalls 512 can beformed in such a manner that an insulating film is newly formed so as tocover the gate insulating films 508 and the electrodes 509, and theinsulating film is partly etched by anisotropic etching in which etchingis performed mainly in a perpendicular direction. By the anisotropicetching, the newly formed insulating film is partly etched to form thesidewalls 512 at the side surfaces of the electrodes 509. Note that thegate insulating films 508 may also be partially etched by theanisotropic etching. The insulating film for forming the sidewalls 512may be a single layer or a stack of layers of one or more of a siliconfilm, a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, and a film containing an organic material such as an organicresin formed by a LPCVD method, a plasma CVD method, a sputteringmethod, or the like. In this embodiment, a 100-nm-thick silicon oxidefilm is formed by a plasma CVD method. As the etching gas, a mixed gasof CHF₃ and helium can be used. Note that the process for forming thesidewalls 512 is not limited to this process.

Next, as shown in FIG. 9B, impurity elements imparting one conductivitytype are added to the semiconductor films 506 and 507 with theelectrodes 509 and the sidewalls 512 used as masks. Note that theimpurity elements imparting the same conductivity type as the impurityelements which have been added in the previous step are added to thesemiconductor films 506 and 507 at a higher concentration than in theprevious step. Note that when the impurity element imparting p-typeconductivity is added to the semiconductor film 507, the semiconductorfilm 506 to which the impurity element imparting n-type conductivity isto be added is covered with a mask or the like so that the impurityelement imparting p-type conductivity is added selectively. In contrast,when the impurity element imparting n-type conductivity is added to thesemiconductor film 506, the semiconductor film 507 to which the impurityelement imparting p-type conductivity is to be added is covered with amask or the like so that the impurity element imparting n-typeconductivity is added selectively.

In addition, by the above-described addition of the impurity elements, apair of high-concentration impurity regions 513, a pair oflow-concentration impurity regions 514, and a channel formation region515 are formed in the semiconductor film 506. Moreover, by the additionof the impurity elements, a pair of high-concentration impurity regions516, a pair of low-concentration impurity regions 517, and a channelformation region 518 are formed in the semiconductor film 507. Thehigh-concentration impurity regions 513 and 516 serve as a source regionor a drain region, and the low-concentration impurity regions 514 and517 serve as LDD (lightly doped drain) regions. Note that the LDDregions are not necessarily provided, and only impurity regions thatserve as source and drain regions may be formed. Alternatively, the LDDregion may be formed on either the source region side or the drainregion side.

Note that in the case of a transistor using silicon, a source region anda drain region respectively serve as a source electrode and a drainelectrode.

Note that the sidewalls 512 formed over the semiconductor film 507 andthe sidewalls 512 formed over the semiconductor film 506 may be formedto have the same width or different widths in the carrier flowdirection. It is preferable that the width of each sidewall 512 over thesemiconductor film 507 which constitutes a part of a p-channeltransistor be larger than the width of each sidewall 512 over thesemiconductor film 506 which constitutes a part of an n-channeltransistor. This is because boron which is added for forming a sourceregion and a drain region in the p-channel transistor is easily diffusedand a short channel effect is easily induced. When the width of eachsidewall 512 in the p-channel transistor is made larger than that ofeach sidewall 512 in the n-channel transistor, boron can be added to thesource region and the drain region at high concentration, and thus theresistance of the source region and the drain region can be reduced.

Next, in order to further reduce the resistance of the source region andthe drain region, silicide is formed in the semiconductor films 506 and507, so that silicide layers may be formed. The silicide is formed insuch a manner that a metal is brought into contact with thesemiconductor films, and silicon in the semiconductor films is made toreact with the metal by heat treatment, a GRTA method, an LRTA method,or the like. The silicide layer may be formed of cobalt silicide ornickel silicide. In the case where the semiconductor films 506 and 507are thin, silicide reaction may proceed to bottoms of the semiconductorfilms 506 and 507 in this region. As a metal material used for theformation of the silicide, the following can be used: titanium (Ti),nickel (Ni), tungsten (W), molybdenum (Mo), cobalt (Co), zirconium (Zr),hafnium (Hf), tantalum (Ta), vanadium (V), neodymium (Nd), chromium(Cr), platinum (Pt), palladium (Pd), or the like. Alternatively, thesilicide may be formed by laser irradiation, light irradiation using alamp, or the like.

Through the above-described steps, an n-channel transistor 520 and ap-channel transistor 521 are formed.

After the step illustrated in FIG. 9B is completed, a transistorincluding an oxide semiconductor is manufactured over the n-channeltransistor 520 and the p-channel transistor 521.

First, as shown in FIG. 10A, an insulating film 530 is formed to coverthe n-channel transistor 520 and the p-channel transistor 521. Byproviding the insulating film 530, oxidation of surfaces of theelectrodes 509 can be prevented when heat treatment is performed.Specifically, it is preferable to use silicon nitride, silicon nitrideoxide, silicon oxynitride, aluminum nitride, aluminum oxide, siliconoxide, or the like for the insulating film 530. In this embodiment, asilicon oxynitride film having a thickness of approximately 50 nm isused as the insulating film 530.

Next, as shown in FIG. 10B, an insulating film 531 and an insulatingfilm 532 are formed over the insulating film 530 to cover the n-channeltransistor 520 and the p-channel transistor 521. The insulating films531 and 532 are formed using materials which can withstand a temperatureof heat treatment in a later manufacturing step. Specifically, aninorganic insulating film of silicon oxide, silicon nitride, siliconnitride oxide, silicon oxynitride, aluminum nitride, aluminum nitrideoxide, or the like can be used for the insulating films 531 and 532, forexample.

Note that the insulating films 531 and 532 are stacked over theinsulating film 530 in this embodiment; however, the insulating filmformed over the insulating film 530 may be an insulating film of asingle layer or an insulating layer in which three or more layers arestacked.

A surface of the insulating film 532 may be planarized by a CMP methodor the like.

Next, as illustrated in FIG. 10B, a gate electrode 601 and an electrode602 are formed over the insulating film 532.

The gate electrode 601 and the electrode 602 can be formed with a singlelayer or a stacked layer using one or more conductive films using ametal material such as molybdenum, titanium, chromium, tantalum,tungsten, neodymium, or scandium; an alloy material which includes anyof these metal materials as a main component; or a nitride of any ofthese metals. Note that aluminum or copper can also be used as suchmetal materials as long as aluminum or copper can withstand atemperature of heat treatment performed in a later process. Aluminum orcopper is preferably combined with a refractory metal material so as toprevent a heat resistance problem and a corrosive problem. As therefractory metal material, molybdenum, titanium, chromium, tantalum,tungsten, neodymium, scandium, or the like can be used.

For example, as a two-layer structure of the gate electrode 601 and theelectrode 602, the following structures are preferable: a two-layerstructure in which a molybdenum film is stacked over an aluminum film, atwo-layer structure in which a molybdenum film is stacked over a copperfilm, a two-layer structure in which a titanium nitride film or atantalum nitride film is stacked over a copper film, and a two-layerstructure in which a titanium nitride film and a molybdenum film arestacked. As a three-layer structure of the gate electrode 601 and theelectrode 602, the following structure is preferable: a stackedstructure containing an aluminum film, an alloy film of aluminum andsilicon, an alloy film of aluminum and titanium, or an alloy film ofaluminum and neodymium in a middle layer and any of a tungsten film, atungsten nitride film, a titanium nitride film, and a titanium film in atop layer and a bottom layer.

Further, a light-transmitting oxide conductive film of indium oxide,indium tin oxide, an indium oxide-zinc oxide alloy, zinc oxide, zincaluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, or thelike may be used as the gate electrode 601 and the electrode 602.

The thicknesses of the gate electrode 601 and the electrode 602 are 10nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, aconductive film for the gate electrode is formed to a thickness of 150nm by a sputtering method using a tungsten target, and then theconductive film is processed (patterned) into a desired shape byetching; thus, the gate electrode 601 and the electrode 602 are formed.Note that end portions of the formed gate electrode are preferablytapered, in which case coverage with a gate insulating film stackedthereover is improved. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

Next, as illustrated in FIG. 10C, a gate insulating film 603 is formedover the gate electrode 601 and the electrode 602. The gate insulatingfilm 603 is formed using a film having a single layer or a stacked layerof one or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum nitride film, an aluminum oxynitride film, an aluminumnitride oxide film, a hafnium oxide film, and a tantalum oxide film by aplasma CVD method, a sputtering method, or the like. It is preferablethat the gate insulating film 603 include impurities such as moisture orhydrogen as little as possible. In the case of forming a silicon oxidefilm by a sputtering method, a silicon target or a quartz target is usedas a target, and oxygen or a mixed gas of oxygen and argon is used as asputtering gas.

Here, the oxide semiconductor that is made to be an intrinsicsemiconductor or a substantially intrinsic semiconductor (the oxidesemiconductor that is highly purified) by removal of impurities isextremely sensitive to an interface state and an interface electriccharge; thus, an interface between the highly-purified oxidesemiconductor and the gate insulating film 603 is important. Therefore,the gate insulating film (GI) that is in contact with thehighly-purified oxide semiconductor needs to have a higher quality.

For example, high-density plasma CVD using microwaves (2.45 GHz) ispreferable because a dense high-quality insulating film having highwithstand voltage can be formed. This is because when thehighly-purified oxide semiconductor is closely in contact with thehigh-quality gate insulating film, the interface state can be reducedand interface properties can be favorable.

Needless to say, a different deposition method such as a sputteringmethod or a plasma CVD method can be used as long as a high-qualityinsulating film can be formed as a gate insulating film. Alternatively,an insulating film whose film quality and interface characteristicsbetween the gate insulating film and the oxide semiconductor areimproved by heat treatment performed after formation of the insulatingfilm may be used. In any case, any insulating film can be used as longas film quality as a gate insulating film is high, interface statedensity between the gate insulating film and an oxide semiconductor isdecreased, and a favorable interface can be formed.

The gate insulating film 603 may have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film formed using a silicon oxide film, a silicon oxynitridefilm, or the like having a lower proportion of nitrogen are stacked. Inthat case, the insulating film such as a silicon oxide film or a siliconoxynitride film is formed between the insulating film having a highbarrier property and the oxide semiconductor film. As the insulatingfilm having a high barrier property, a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, an aluminum nitride oxidefilm, or the like can be given, for example. The insulating film havinga high barrier property is used, so that impurities in an atmosphere,such as moisture or hydrogen, or impurities included in the substrate,such as an alkali metal or a heavy metal, can be prevented from enteringthe oxide semiconductor film, the gate insulating film 603, or theinterface between the oxide semiconductor film and another insulatingfilm and the vicinity thereof. In addition, the insulating film having alower proportion of nitrogen such as a silicon oxide film or a siliconoxynitride film is formed so as to be in contact with the oxidesemiconductor film, so that the insulating film having a high barrierproperty can be prevented from being in direct contact with the oxidesemiconductor film.

For example, a stacked-layer film with a thickness of 100 nm may beformed as the gate insulating film 603 as follows: a silicon nitridefilm (SiN_(y) (y>0)) with a thickness of greater than or equal to 50 nmand less than or equal to 200 nm is formed by a sputtering method as afirst gate insulating film, and a silicon oxide film (SiO_(x) (x>0))with a thickness of greater than or equal to 5 nm and less than or equalto 300 nm is stacked over the first gate insulating film as a secondgate insulating film. The thickness of the gate insulating film 603 maybe set as appropriate depending on the desired characteristics of thetransistor. The thickness may be approximately 350 nm to 400 nm.

In this embodiment, the gate insulating film 603 is formed to have astructure in which a 100-nm-thick silicon oxide film formed by asputtering method is stacked over a 50-nm-thick silicon nitride filmformed by a sputtering method.

Note that in order that hydrogen, hydroxyl group, and moisture arecontained as little as possible in the gate insulating film 603, it ispreferable that the base substrate 503 over which the gate electrode 601and the electrode 602 are formed be preheated in a preheating chamber ofthe sputtering apparatus, so that impurities such as moisture orhydrogen adsorbed onto the base substrate 503 is eliminated andevacuated, as pretreatment before film formation. The temperature forthe preheating is higher than or equal to 100° C. and lower than orequal to 400° C., preferably higher than or equal to 150° C. and lowerthan or equal to 300° C. As an evacuation unit provided in thepreheating chamber, a cryopump is preferable. Note that this preheatingtreatment can be omitted.

Next, over the gate insulating film 603, an oxide semiconductor filmhaving a thickness of greater than or equal to 2 nm and less than orequal to 200 nm, preferably greater than or equal to 3 nm and less thanor equal to 50 nm, more preferably greater than or equal to 3 nm andless than or equal to 20 nm is formed. The oxide semiconductor film isformed by a sputtering method using an oxide semiconductor as a target.Moreover, the oxide semiconductor film can be formed by a sputteringmethod under a rare gas (for example, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere including a rare gas (for example,argon) and oxygen.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust attached to a surface of the gate insulating film 603 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering means amethod in which a surface of a substrate is modified by application ofvoltage to the substrate side, not to a target side, in an argonatmosphere with the use of an RF power source and by generating plasmain the vicinity of the substrate. Note that instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, or the like maybe used. Alternatively, an argon atmosphere to which oxygen, nitrousoxide, or the like is added may be used. Alternatively, an argonatmosphere to which chlorine, carbon tetrafluoride, or the like is addedmay be used.

The above oxide semiconductor can be used for the oxide semiconductorfilm.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-basednon-single-crystal film with a thickness of 30 nm, which is obtained bya sputtering method using a metal oxide target including indium (In),gallium (Ga), and zinc (Zn), is used. As the target, for example, ametal oxide target having a composition ratio of metals,In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. Thetarget may contain SiO₂ at greater than or equal to 2 wt % and less thanor equal to 10 wt %. The fill rate of the metal oxide target includingIn, Ga, and Zn is higher than or equal to 90% and lower than or equal to100%, preferably higher than or equal to 95% and lower than or equal to99.9%. With use of a metal oxide target with high fill rate, thedeposited oxide semiconductor film has high density.

In this embodiment, the oxide semiconductor film is formed over the basesubstrate 503 as follows: the substrate is held in a treatment chamberwith pressure reduced, moisture remaining in the treatment chamber isbeing removed, a sputtering gas from which hydrogen and moisture areremoved is introduced, and the above-described target is used. At thattime, the substrate may be heated at higher than or equal to 100° C. andlower than or equal to 600° C., preferably higher than or equal to 200°C. and lower than or equal to 400° C. Film formation is performed whilethe substrate is heated, whereby the impurity concentration in the oxidesemiconductor film formed can be reduced. In addition, damage bysputtering can be reduced. In order to remove moisture remaining in thetreatment chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the treatment chamber which is evacuated with thecryopump, for example, a hydrogen atom, a compound containing a hydrogenatom, such as water (H₂O), (more preferably, also a compound containinga carbon atom), and the like are removed, whereby the impurityconcentration in the oxide semiconductor film formed in the treatmentchamber can be reduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, the directcurrent (DC) power source is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of the oxygen flow rate is 100%). Note that apulse direct current (DC) power source is preferable because powdersubstances called particles generated in film formation can be reducedand the film thickness can be uniform.

Note that in order that hydrogen, hydroxyl group, and moisture arecontained as little as possible in the oxide semiconductor film, it ispreferable that the base substrate 503 over which layers up to andincluding the gate insulating film 603 are formed be preheated in apreheating chamber of the sputtering apparatus, so that impurities suchas hydrogen or moisture adsorbed onto the base substrate 503 iseliminated and evacuated, as pretreatment before film formation. Thetemperature for the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably higher than or equal to 150°C. and lower than or equal to 300° C. As an evacuation unit provided inthe preheating chamber, a cryopump is preferable. Note that thispreheating treatment can be omitted. Further, this preheating may besimilarly performed on the base substrate 503 over which layers up toand including a source electrode 607 and a drain electrode 608, wirings609 to 611 are formed, before the formation of an insulating film 612.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a deposition method using a sputtering method, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, or a bias sputtering method in whichvoltage is also applied to a substrate during deposition can be used.

The gate insulating film 603 and the oxide semiconductor film may beformed successively without exposure to air. Successive film formationwithout exposure to air makes it possible to obtain each interfacebetween stacked layers, which is not contaminated by atmosphericcomponents or impurity elements floating in air, such as water,hydrocarbon, or the like. Therefore, variation in characteristics of thetransistor can be reduced.

Next, as illustrated in FIG. 10C, the oxide semiconductor film isprocessed (patterned) into a desired shape by etching or the like,whereby an island-shaped oxide semiconductor film 605 is formed over thegate insulating film 603 in a position where the island-shaped oxidesemiconductor film 605 overlaps with the gate electrode 601.

A resist mask for forming the island-shaped oxide semiconductor film 605may be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

Note that the etching for forming the island-shaped oxide semiconductorfilm 605 may be dry etching, wet etching, or both dry etching and wetetching. As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused. Alternatively, a gas containing fluorine (fluorine-based gas suchas carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into a desired shape, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, a solution obtained by mixingphosphoric acid, acetic acid, and nitric acid, an ammonia peroxidemixture (hydrogen peroxide water at 31 wt %:ammonia water at 28 wt%:water=5:2:2), or the like can be used. Alternatively, ITO-07N(manufactured by Kanto Chemical Co., Inc.) may be used. The etchantafter the wet etching is removed together with the etched materials bycleaning. The waste liquid including the etchant and the material etchedoff may be purified and the material may be reused. When a material suchas indium contained in the oxide semiconductor film is collected fromthe waste liquid after the etching and reused, the resources can beefficiently used and the cost can be reduced.

Note that it is preferable that reverse sputtering be performed beforethe formation of a conductive film in a subsequent step so that a resistresidue or the like that attaches onto surfaces of the island-shapedoxide semiconductor film 605 and the gate insulating film 603 isremoved.

Next, the oxide semiconductor film 605 is subjected to heat treatmentunder a reduced-pressure atmosphere, an inert gas atmosphere such as anitrogen atmosphere or a rare gas atmosphere, an oxygen atmosphere, oran ultra-dry air (the moisture amount is 20 ppm (−55° C. by conversioninto a dew point) or less, preferably 1 ppm or less, more preferably 10ppb or less, in the case where measurement is performed by a dew pointmeter in a cavity ring down laser spectroscopy (CRDS) method). Byperforming the heat treatment on the oxide semiconductor film 605,moisture or hydrogen in the oxide semiconductor film 605 can beeliminated. Specifically, the heat treatment may be performed at higherthan or equal to 300° C. and lower than or equal to 850° C. (or atemperature lower than or equal to the strain point of the glasssubstrate), preferably higher than or equal to 550° C. and lower than orequal to 750° C. For example, the heat treatment may be performed at600° C. for greater than or equal to 3 minutes and less than or equal to6 minutes. With an RTA method for the heat treatment, dehydration ordehydrogenation can be performed in a short time; therefore, treatmentcan be performed even at a temperature higher than the strain point of aglass substrate. Alternatively, the heat treatment may be performed inthe state where the substrate temperature is 450° C. for approximately 1hour.

In this embodiment, heat treatment is performed on the oxidesemiconductor film 605 at a substrate temperature of 600° C. for 6minutes in a nitrogen atmosphere with the use of an electrical furnacethat is one of heat treatment apparatuses, and then, the oxidesemiconductor film is not exposed to air and water or hydrogen isprevented from entering the oxide semiconductor film.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and a heat treatment apparatus may be provided with a devicefor heating an object to be processed by heat conduction or heatradiation from a heating element such as a resistance heating element.For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. An LRTA apparatus is an apparatus for heating anobject to be processed by radiation of light (an electromagnetic wave)emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenonarc lamp, a carbon arc lamp, a high pressure sodium lamp, or a highpressure mercury lamp. A GRTA apparatus is an apparatus for heattreatment using a high-temperature gas. As the gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas such as argon is used.

For example, the heat treatment can employ GRTA in which the substrateis moved into an inert gas heated at a high temperature of 650° C. to700° C., and heated for several minutes there, and then the substrate ismoved out of the high-temperature inert gas. With GRTA, high-temperatureheat treatment for a short period of time can be achieved.

Note that in the heat treatment, it is preferable that moisture,hydrogen, or the like be not contained in nitrogen or a rare gas such ashelium, neon, or argon. It is preferable that the purity of nitrogen orthe rare gas such as helium, neon, or argon which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

When impurities such as moisture or hydrogen is added to an oxidesemiconductor, in a gate bias-temperature stress test (BT test, the testcondition is, for example, at 85° C. with 2×10⁶ V/cm for 12 hours), abond between the impurities and a main component of the oxidesemiconductor is broken by a high electrical field (B: bias) and hightemperature (T: temperature), and a dangling bond generated causes driftof the threshold voltage (Vth). However, in the above-described manner,by improving the interface characteristics between the gate insulatingfilm and the oxide semiconductor film and removing impurities,particularly hydrogen, moisture, or the like, in the oxide semiconductorfilm as much as possible, a transistor which remains stable even withrespect to the BT test can be obtained.

Through the above-described steps, the concentration of hydrogen in theoxide semiconductor film 605 can be reduced and the island-shaped oxidesemiconductor film is highly purified. Thus, the oxide semiconductorfilm can be stabilized. In addition, heat treatment at a temperature oflower than or equal to the glass transition temperature makes itpossible to form an oxide semiconductor film with a wide band gap inwhich carrier density is extremely low. Therefore, a transistor can bemanufactured using a large-sized substrate, so that productivity can beincreased. In addition, by using the oxide semiconductor film in whichthe hydrogen concentration is reduced and purity is improved, it ispossible to manufacture a transistor with high withstand voltage, areduced short-channel effect, and a high on-off ratio.

Note that in the case where the oxide semiconductor film is heated,although depending on a material of the oxide semiconductor film orheating conditions, plate-shaped crystals are formed over the topsurface of the oxide semiconductor film in some cases. The plate-shapedcrystals preferably form a single crystal body in which crystals arec-axis-oriented in a direction generally perpendicular to the surface ofthe oxide semiconductor film. Even when the plate-shaped crystals do notform a single crystal body, the plate-shaped crystals preferably form apolycrystal body in which each crystal is c-axis-oriented in a directiongenerally perpendicular to the surface of the oxide semiconductor film.In the above-described polycrystal body, in addition to the c-axisorientation, the crystals preferably have identical a-b planes, a-axes,or b-axes. Note that in the case where a surface of the base of theoxide semiconductor film is uneven, the plate-shaped crystals form apolycrystal body. Therefore, it is preferable that the surface of thebase is as planar as possible.

Next, the insulating film 530, the insulating film 531, the insulatingfilm 532, and the gate insulating film 603 are partly etched, so thatcontact holes reaching the high-concentration impurity regions 513included in the n-channel transistor 520, contact holes reaching thehigh-concentration impurity regions 516 included in the p-channeltransistor 521, and a contact hole reaching the electrode 602 areformed. Then, a conductive film used for the source electrode or thedrain electrode (including a wiring formed in the same layer as thesource electrode or the drain electrode) is formed over the oxidesemiconductor film 605 by a sputtering method or a vacuum evaporationmethod, and then the conductive film is patterned by etching or thelike, whereby as illustrated in FIG. 11A, the source electrode 607 andthe drain electrode 608 over the oxide semiconductor film 605, a wiring609 in contact with the electrode 602 and the high-concentrationimpurity region 513, a wiring 610 in contact with the high-concentrationimpurity region 516, and a wiring 611 in contact with thehigh-concentration impurity region 513 and the high-concentrationimpurity region 516 are formed.

As the material of the conductive film to be the source electrode andthe drain electrode (including a wiring formed in the same layer as thesource electrode and the drain electrode), there are an element selectedfrom Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy including any of the aboveelements as a component; an alloy film any of these elements incombination; and the like. Further, a structure may be employed in whicha film formed using a high-melting point metal such as Cr, Ta, Ti, Mo,or W is stacked over either the lower side or the upper side of a metalfilm such as Al or Cu. Still alternatively, when an Al material to whichan element preventing generation of hillocks and whiskers in an Al film,such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Y, is added is used, heatresistance can be increased.

Further, the conductive film may have a single-layer structure or alayered structure of two or more layers. For example, a single-layerstructure of an aluminum film including silicon, a two-layer structurein which a titanium film is stacked over an aluminum film, a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order, and the like can be given.

Alternatively, the conductive film to be the source and drain electrodes(including a wiring formed in the same layer as the source and drainelectrodes) may be formed using a conductive metal oxide. As theconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), indium tin oxide (In₂O₃—SnO₂, abbreviated to ITO), an alloyof indium oxide and zinc oxide (In₂O₃—ZnO), or any of the metal oxidematerials containing silicon or silicon oxide can be used.

In the case where heat treatment is performed after formation of theconductive film, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor film 605 is not removed inetching of the conductive film as much as possible. Depending on theetching conditions, an exposed portion of the island-shaped oxidesemiconductor film 605 is partly etched, so that an island-shaped oxidesemiconductor film 605 having a groove (a recessed portion) is formed insome cases.

In this embodiment, since the titanium film is used as the conductivefilm, wet etching can be selectively performed on the conductive filmwith the use of an ammonia peroxide mixture (hydrogen peroxide water at31 wt %:ammonia water at 28 wt %:water=5:2:2), in which case part of theoxide semiconductor film 605 is also etched in some cases.Alternatively, dry etching may be performed on the conductive film withthe use of a gas containing chlorine (Cl₂), boron chloride (BCl₃), orthe like.

In order to reduce the number of photomasks and steps in aphotolithography process, etching may be performed with the use of aresist mask formed using a multi-tone mask which is a light-exposuremask through which light is transmitted so as to have a plurality ofintensities. A resist mask formed with the use of a multi-tone mask hasa plurality of thicknesses and further can be changed in shape byetching; therefore, the resist mask can be used in a plurality ofetching steps for processing into different patterns. Therefore, aresist mask corresponding to at least two kinds or more of differentpatterns can be formed by one multi-tone mask. Thus, the number oflight-exposure masks can be reduced and the number of correspondingphotolithography process can be also reduced, whereby simplification ofa process can be realized.

Next, plasma treatment is performed using a gas such as N₂O, N₂, or Ar.By the plasma treatment, water or the like which attaches or is adsorbedto an exposed surface of the oxide semiconductor film is removed. Plasmatreatment may be performed using a mixture gas of oxygen and argon aswell.

After the plasma treatment, as illustrated in FIG. 11B, the insulatingfilm 612 is formed so as to cover the source electrode 607, the drainelectrode 608, the wirings 609 to 611, and the oxide semiconductor film605. The insulating film 612 preferably contains impurities such asmoisture or hydrogen as little as possible, and the insulating film 612may be formed using a single-layer insulating film or a plurality ofinsulating films stacked. When hydrogen is contained in the insulatingfilm 612, entry of the hydrogen to the oxide semiconductor film, orextraction of oxygen in the oxide semiconductor film by hydrogen mayoccur, thereby causing a backchannel portion of the oxide semiconductorfilm to have lower resistance (to be n-type), so that a parasiticchannel may be formed. Therefore, it is preferable that a formationmethod in which hydrogen is not used is employed in order to form theinsulating film 612 containing hydrogen as little as possible. Amaterial having a high barrier property is preferably used for theinsulating film 612. For example, as the insulating film having a highbarrier property, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, an aluminum nitride oxide film, or the likecan be used. When a plurality of insulating films stacked is used, aninsulating film having a lower proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film is formed on the side closer tothe oxide semiconductor film 605 than the insulating film having a highbarrier property. Then, the insulating film having a high barrierproperty is formed so as to overlap with the source electrode 607, thedrain electrode 608, and the oxide semiconductor film 605 with theinsulating film having a lower proportion of nitrogen between theinsulating film having a high barrier property and the source electrode607, the drain electrode 608, and the island-shaped oxide semiconductorfilm 605. When the insulating film having a high barrier property isused, impurities such as moisture or hydrogen can be prevented fromentering the oxide semiconductor film 605, the gate insulating film 603,or the interface between the oxide semiconductor film 605 and anotherinsulating film and the vicinity thereof. In addition, the insulatingfilm having a lower proportion of nitrogen such as a silicon oxide filmor a silicon oxynitride film is formed so as to be in contact with theoxide semiconductor film 605, so that the insulating film formed using amaterial having a high barrier property can be prevented from being indirect contact with the oxide semiconductor film 605.

In this embodiment, the insulating film 612 having a structure in whicha silicon nitride film having a thickness of 100 nm formed with asputtering method is stacked over a silicon oxide film having athickness of 200 nm formed with a sputtering method is formed. Thesubstrate temperature in film formation may be higher than or equal toroom temperature and lower than or equal to 300° C. and in thisembodiment, is 100° C.

Note that after the insulating film 612 is formed, heat treatment may beperformed. The heat treatment is preferably performed in areduced-pressure atmosphere, an inert gas atmosphere such as a nitrogenatmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or anultra dry air atmosphere (the moisture amount is 20 ppm (−55° C. byconversion into a dew point) or less, preferably 1 ppm or less, morepreferably 10 ppb or less, in the case where measurement is performed bya dew point meter in a cavity ring down laser spectroscopy (CRDS)method) at higher than or equal to 200° C. and lower than or equal to400° C., for example, higher than or equal to 250° C. and lower than orequal to 350° C. In this embodiment, heat treatment for 1 hour at 250°C. in a nitrogen atmosphere is performed. Alternatively, RTA treatmentfor a short time at a high temperature may be performed before theformation of the source electrode 607, the drain electrode 608, and thewirings 609 to 611 in a manner similar to the heat treatment performedon the oxide semiconductor film. Even when oxygen deficiency isgenerated in the oxide semiconductor film 605 owing to the pervious heattreatment on the oxide semiconductor film, by performing heat treatmentafter the insulating film 612 containing oxygen is formed in contactwith the exposed region of the oxide semiconductor film 605 providedbetween the source electrode 607 and the drain electrode 608, oxygen issupplied to the oxide semiconductor film 605. By supplying oxygen to theregion of the oxide semiconductor film 605 in contact with theinsulating film 612, oxygen deficiency that serves as a donor is reducedand the stoichiometric composition ratio can be satisfied. As a result,the oxide semiconductor film 605 can be made to be an i-typesemiconductor film or a substantially i-type semiconductor film.Accordingly, electric characteristics of the transistor can be improvedand variation in the electric characteristics thereof can be reduced.The timing of this heat treatment is not particularly limited as long asit is after the formation of the insulating film 612, and this heattreatment can be performed without increasing the number ofmanufacturing steps by doubling as another step such as heat treatmentfor formation of a resin film or heat treatment for reduction of theresistance of a transparent conductive film, so that the oxidesemiconductor film 605 can be made to be an i-type semiconductor film ora substantially i-type semiconductor film.

Next, after a conductive film is formed over the insulating film 612, aback gate electrode may be formed so as to overlap with the oxidesemiconductor film 605 by patterning the conductive film. When the backgate electrode is formed, an insulating film is formed so as to coverthe back gate electrode. The back gate electrode can be formed using amaterial and a structure similar to those of the gate electrode 601, theelectrode 602, the source electrode 607, the drain electrode 608, thewiring 609, the wiring 610, or the wiring 611.

The thickness of the back gate electrode is 10 nm to 400 nm, preferably100 nm to 200 nm. In this embodiment, the back gate electrode may beformed in a such a manner that a conductive film in which a titaniumfilm, an aluminum film, and a titanium film are stacked is formed, aresist mask is formed by a photolithography method or the like, andunnecessary portions are removed by etching so that the conductive filmis processed (patterned) into a desired shape.

The insulating film is preferably formed using a material having a highbarrier property which can prevent moisture, hydrogen, oxygen, and thelike in an atmosphere from influencing characteristics of thetransistor. For example, it is possible to form a single-layer structureor a stacked-layer structure of a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, an aluminum nitride oxidefilm, or the like, as an insulating film having a high barrier property,by a plasma CVD method, a sputtering method, or the like. In order toobtain an effect of a barrier property, the insulating film ispreferably formed to a thickness of 15 nm to 400 nm, for example.

In this embodiment, an insulating film is formed to a thickness of 300nm by a plasma CVD method. The insulating film is formed under thefollowing conditions: the silane gas flow rate is 4 sccm; the flow rateof dinitrogen monoxide (N₂O) is 800 sccm; and the substrate temperatureis 400° C.

Through the above-described steps, a transistor 620 that functions as aswitching element, the n-channel transistor 520 and the p-channeltransistor 521 that form a phase-inversion element, and a capacitor 623are formed. Note that the capacitor 623 is formed in a region where theelectrode 602 and the source electrode 607 overlap with each other withthe gate insulating film 603 interposed therebetween. Note also that thecapacitor 623 is not necessarily formed over the same layer as thetransistor 620; for example, the capacitor 623 may be formed over thesame layer as the n-channel transistor 520 and the p-channel transistor521.

The transistor 620 includes the gate electrode 601, the gate insulatingfilm 603 over the gate electrode 601, the oxide semiconductor film 605which is over the gate insulating film 603 and overlaps with the gateelectrode 601, and a pair of the source electrode 607 and the drainelectrode 608 formed over the oxide semiconductor film 605. Thetransistor 620 may further include the insulating film 612 provided overthe oxide semiconductor film 605 as its component. Note that thetransistor 620 illustrated in FIG. 11B has a channel-etched structure inwhich part of the oxide semiconductor film 605 is exposed between thesource electrode 607 and the drain electrode 608.

Although description is given using a single-gate transistor as thetransistor 620, a multi-gate transistor including a plurality of channelformation regions by including a plurality of the gate electrodes 601that are electrically connected to each other may be formed as needed.

Note that the energy band gap of the oxide semiconductor is 3 eV to 3.5eV. The band gap of silicon carbide and the band gap of gallium nitrideare 3.26 eV and 3.39 eV, respectively, which are about three times aslarge as that of silicon. Therefore, these compound semiconductors suchas silicon carbide and gallium nitride are similar to the oxidesemiconductor in that they are both wide band gap semiconductors. Thecharacteristics of the wide band gap is advantageous for improving thewithstand voltage, reducing the electric power loss of a semiconductordevice, and the like.

However, the compound semiconductor such as silicon carbide or galliumnitride needs much higher process temperature or treatment temperaturethan the oxide semiconductor. The process temperature of silicon carbideis about 1500° C. and the process temperature of gallium nitride isabout 1100° C., which does not allow film formation over a silicon waferthat can be obtained easily or a glass substrate whose allowabletemperature limit is low. Therefore, an inexpensive substrate cannot beused, and further, the compound semiconductor cannot be applied when thesize of a substrate is increased, so that the mass productivity ofsemiconductor devices using the compound semiconductor such as siliconcarbide or gallium nitride is low. In contrast, the oxide semiconductorcan be deposited in heat treatment at 300° C. to 850° C. so as to bedeposited over a glass substrate. In addition, as described in thisembodiment, it is possible to form a semiconductor element using anoxide semiconductor over an integrated circuit using a normalsemiconductor material.

Next, how characteristics of the transistor are influenced by highpurification of the oxide semiconductor film by removal of impuritiessuch as moisture or hydrogen contained in the oxide semiconductor filmas much as possible as in this embodiment will be described.

FIG. 19 is a cross-sectional view of a transistor including an oxidesemiconductor. An oxide semiconductor film (OS) is provided over a gateelectrode (GE) with a gate insulating film (GI) therebetween, and asource electrode (S) and a drain electrode (D) are provided thereover.An insulating film is formed over the source electrode (S) and the drainelectrode (D).

FIG. 20 is an energy band diagram (a schematic diagram) along an A-A′section illustrated in FIG. 19. In FIG. 20, a black circle (•) and awhite circle (∘) represent an electron and a hole and have electriccharges (−q, +q), respectively. The case where a positive voltage(V_(D)>0) is applied to the drain electrode (D) and no voltage (V_(G)=0)is applied to the gate electrode (GE) is shown by dashed lines, and thecase where a positive voltage (V_(D)>0) is applied to the drainelectrode (D) and a positive voltage (V_(G)>0) is applied to the gateelectrode (GE) is shown by solid lines. In the case where voltage is notapplied to the gate electrode (GE), a carrier (an electron) is notinjected to the oxide semiconductor film (OS) side from the sourceelectrode (S) owing to the high potential barrier, so that current doesnot flow, which means an OFF state. In contrast, when a positive voltageis applied to the gate electrode (GE), the potential barrier isdecreased, so that current flows in the oxide semiconductor film (OS),which means an ON state.

FIGS. 21A and 21B are energy band diagrams (schematic diagrams) along aB-B′ section illustrated in FIG. 19. FIG. 21A illustrates a state wherea positive voltage (V_(G)>0) is applied to a gate electrode (GE) and anON state where carriers (electrons) flow between the source electrodeand the drain electrode. FIG. 21B illustrates a state where a negativevoltage (V_(G)<0) is applied to the gate electrode (GE) and thetransistor is in an OFF state.

FIG. 22 illustrates relations between the vacuum level and the workfunction (φ_(M)) of a metal and between the vacuum level and theelectron affinity (χ) of an oxide semiconductor.

At normal temperature, electrons in the metal are degenerated and theFermi level is located in the conduction band. On the other hand, ingeneral, a conventional oxide semiconductor is an n-type semiconductor,and the Fermi level (Ef) thereof is located nearer the conduction band(Ec) away from an intrinsic Fermi level (Ei) which is located in thecenter of the band gap. Note that it is known that part of hydrogen inthe oxide semiconductor serves as a donor and one of factors that makethe oxide semiconductor an n-type semiconductor. In addition, oxygendeficiency is also known as one of the factors that make the oxidesemiconductor have n-type conductivity.

In contrast, in one embodiment of the present invention, an oxidesemiconductor is made to be an intrinsic (i-type) semiconductor orextremely close to an intrinsic semiconductor by removing hydrogen thatis an n-type impurity from an oxide semiconductor, highly purifying theoxide semiconductor such that an impurity other than a main component ofthe oxide semiconductor is prevented from being contained therein asmuch as possible, and removing oxygen deficiency. That is, the oxidesemiconductor becomes an i-type semiconductor not by addition ofimpurities but by removal of impurities such as moisture or hydrogen andoxygen deficiency as much as possible to have high purity, so that anoxide semiconductor which is an intrinsic (i-type) semiconductor or isextremely close to an intrinsic (i-type) semiconductor is obtained. Withthe above structure, the Fermi level (Ef) can be extremely close to thesame level as the intrinsic Fermi level (Ei), as indicated by arrows.

It is said that the band gap (Eg) of an oxide semiconductor is 3.15 eVand that the electron affinity (χ) thereof is 4.3 V. In the case wheretitanium (Ti) is used as a material for forming the source electrode andthe drain electrode, the work function of titanium (Ti) is substantiallyequal to the electron affinity (χ) of the oxide semiconductor. In thatcase, a Schottky barrier to electrons is not formed at an interfacebetween the metal and the oxide semiconductor. There are materials thatsatisfy this condition besides titanium.

In this case, as shown in FIG. 21A, the electron moves along the lowestpart of the oxide semiconductor, which is energetically stable, at aninterface between the gate insulating film and the highly-purified oxidesemiconductor.

In FIG. 21B, when a negative voltage is applied to the gate electrode(GE), holes which are minority carriers are substantially zero;therefore, current is extremely close to zero.

For example, even when an element has a channel width (W) of 1×10⁶ μmand a channel length (L) of 10 μm, OFF-state current can be less than orequal to the measurement limit of a semiconductor parameter analyzer,i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage)between the source electrode and the drain electrode of from 1 V to 10V. According to measurement, a transistor can have a much lowerOFF-state current density, which is from 10 zA/μm to 100 zA/μm, at avoltage between a source electrode and a drain electrode of thetransistor of 3 V. In the measurement, a transistor including ahighly-purified oxide semiconductor film and a 100-nm-thick gateinsulating film was used for a switching element which stores charge ofa storage capacitor, and the OFF-state current of the transistor wasmeasured by a change in the amount of charge in the storage capacitorper a unit hour. That is, the OFF-state current density of thetransistor in which the highly-purified oxide semiconductor film is usedas an active layer can be less than or equal to 100 zA/μm, preferablyless than or equal to 10 zA/μm, more preferably less than or equal to 1zA/μm.

In this manner, the oxide semiconductor film is highly purified so thatimpurities such as moisture or hydrogen other than a main component ofthe oxide semiconductor are contained as little as possible, whereby theoperation of the transistor can be favorable.

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

Embodiment 5

In this embodiment, a transistor including an oxide semiconductor filmhas a structure which is different from that of the transistor ofEmbodiment 4 is described.

In a manner similar to that in Embodiment 4, a memory device illustratedin FIG. 12A includes the n-channel transistor 520 and the p-channeltransistor 521 each of which includes crystalline silicon. In addition,a bottom-gate transistor 630 which has a channel-protective structureand includes an oxide semiconductor film is formed over the n-channeltransistor 520 and the p-channel transistor 521 in FIG. 12A.

The transistor 630 includes a gate electrode 631 provided over theinsulating film 532, a gate insulating film 632 provided over the gateelectrode 631, an oxide semiconductor film 633 which overlaps with thegate electrode 631 over the gate insulating film 632, a channelprotective film 634 which is provided over the island-shaped oxidesemiconductor film 633 to overlap with the gate electrode 631, and asource electrode 635 and a drain electrode 636 which are provided overthe oxide semiconductor film 633. The transistor 630 may further includean insulating film 637 provided over the oxide semiconductor film 633 asits component.

The channel protective film 634 can prevent a portion of the oxidesemiconductor film 633, which serves as a channel formation region, frombeing damaged in a later step (for example, reduction in thickness dueto plasma or an etchant in etching). Therefore, reliability of thetransistor can be improved.

An inorganic material containing oxygen (silicon oxide, silicon nitrideoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, or thelike) can be used for the channel protective film 634. The channelprotective film 634 can be formed by a vapor deposition method such as aplasma CVD method or a thermal CVD method, or a sputtering method. Afterthe formation of the channel protective film 634, the shape thereof isprocessed by etching. Here, the channel protective film 634 is formed insuch a manner that a silicon oxide film is formed by a sputtering methodand processed by etching using a mask formed by photolithography.

By using the inorganic material containing oxygen for the channelprotective film 634, even when oxygen deficiency is generated in theoxide semiconductor film 633 owing to heat treatment for reducingmoisture or hydrogen, a region of the oxide semiconductor film 633 whichis in contact with at least the channel protective film 634 can besupplied with oxygen and oxygen deficiency which serves as a donor canbe reduced, so that a structure which satisfies the stoichiometriccomposition ratio can be obtained. Therefore, the channel formationregion can be made to be an i-type semiconductor or a substantiallyi-type semiconductor, variation in electric characteristics of thetransistor due to oxygen deficiency can be reduced, and the electriccharacteristics can be improved.

Note that the transistor 630 may further include a back gate electrodeover the insulating film 637. The back gate electrode is formed so as tooverlap with a channel formation region in the oxide semiconductor film633. Further, the back gate electrode may be electrically insulated andin a floating state, or may be in a state where the back gate electrodeis supplied with a potential. In the latter case, the back gateelectrode may be supplied with the potential having the same level asthe gate electrode 631, or may be supplied with a fixed potential suchas a ground potential. The level of the potential supplied to the backgate electrode is controlled, whereby the threshold voltage of thetransistor 630 can be controlled.

In a manner similar to that in Embodiment 4, a memory device illustratedin FIG. 12B includes the n-channel transistor 520 and the p-channeltransistor 521 each of which includes crystalline silicon. In addition,a bottom-contact transistor 640 including an oxide semiconductor film isformed over the n-channel transistor 520 and the p-channel transistor521 in FIG. 12B.

The transistor 640 includes a gate electrode 641 provided over theinsulating film 532, a gate insulating film 642 provided over the gateelectrode 641, a source electrode 643 and a drain electrode 644 whichare provided over the gate insulating film 642, and an oxidesemiconductor film 645 which overlaps with the gate electrode 641. Thetransistor 640 may further include an insulating film 646 provided overthe oxide semiconductor film 645 as its component.

In the case of the bottom-contact transistor 640 as illustrated in FIG.12B, the thickness of each of the source electrode 643 and the drainelectrode 644 is preferably smaller than that of the bottom-gatetransistor described in Embodiment 4 in order to prevent disconnectionof the oxide semiconductor film 645 formed later. Specifically, thethicknesses of the source electrode 643 and the drain electrode 644 are10 nm to 200 nm, preferably 50 nm to 75 nm.

Note that the transistor 640 may further include a back gate electrodeover the insulating film 646. The back gate electrode is formed so as tooverlap with a channel formation region in the oxide semiconductor film645. Further, the back gate electrode may be electrically insulated andin a floating state, or may be in a state where the back gate electrodeis supplied with a potential. In the latter case, the back gateelectrode may be supplied with the potential having the same level asthe gate electrode 641, or may be supplied with a fixed potential suchas a ground potential. The level of the potential supplied to the backgate electrode is controlled, whereby the threshold voltage of thetransistor 640 can be controlled.

In a manner similar to that in Embodiment 4, a memory device illustratedin FIG. 12C includes the n-channel transistor 520 and the p-channeltransistor 521 each of which includes crystalline silicon. In addition,a top-gate transistor 650 including an oxide semiconductor film isprovided over the n-channel transistor 520 and the p-channel transistor521 in FIG. 12C.

The transistor 650 includes a source electrode 651 and a drain electrode652 which are provided over the insulating film 532, an oxidesemiconductor film 653 which is provided over the source electrode 651and the drain electrode 652, a gate insulating film 654 provided overthe oxide semiconductor film 653, and a gate electrode 655 whichoverlaps with the oxide semiconductor film 653 over the gate insulatingfilm 654. In addition, the transistor 650 may further include aninsulating film 656 provided over the gate electrode 655 as itscomponent.

In the case of the top-gate transistor 650 as illustrated in FIG. 12C,the thickness of each of the source electrode 651 and the drainelectrode 652 is preferably smaller than that of the bottom-gatetransistor described in Embodiment 4 in order to prevent disconnectionof the oxide semiconductor film 653 formed later. Specifically, thethicknesses of the source electrode 651 and the drain electrode 652 are10 nm to 200 nm, preferably 50 nm to 75 nm.

In the memory device which is one embodiment of the present invention, aphase-inversion element, a switching element, and the like may include atransistor which is manufactured using a bulk single crystalsemiconductor substrate. In FIG. 23, a cross-sectional view of a memorydevice in which a transistor including an oxide semiconductor is formedover a transistor which is formed using the bulk single crystalsemiconductor substrate is illustrated as an example.

The memory device illustrated in FIG. 23 includes an n-channeltransistor 661 and a p-channel transistor 662 which are provided over asemiconductor substrate 660, a transistor 664 which is provided over aninsulating film 663 covering the n-channel transistor 661 and thep-channel transistor 662 and which is used as a capacitor switchingelement, and a capacitor 665.

Although the case where the transistor 664 includes an oxidesemiconductor in a channel formation region and has a structuredescribed in Embodiment 4 is described as an example, the structure maybe that illustrated in FIGS. 12A to 12C.

The semiconductor substrate 660 can be, for example, a single crystalsilicon substrate having n-type or p-type conductivity, a compoundsemiconductor substrate (e.g., a GaAs substrate, an InP substrate, a GaNsubstrate, a SiC substrate, a sapphire substrate, or a ZnSe substrate),or the like. In FIG. 23, the case where a single crystal siliconsubstrate having n-type conductivity is used is illustrated as anexample.

In addition, the n-channel transistor 661 and the p-channel transistor662 are electrically isolated by an element isolation insulating film666. For formation of the element isolation insulating film 666, aselective oxidation method (local oxidation of silicon (LOCOS) method),a trench isolation method, or the like can be used.

In a region where the p-channel transistor 662 is formed, a regioncalled a p-well 667 is formed by selectively introducing an impurityelement imparting p-type conductivity. In the case where a semiconductorsubstrate having p-type conductivity is used, an impurity elementimparting n-type conductivity may be selectively introduced to a regionwhere the n-channel transistor 661 is formed, so that an n-well may beformed.

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

Embodiment 6

In this embodiment, a structure of a memory device which is oneembodiment of the present invention will be described. In the memotydevice, a transistor including a highly-purified oxide semiconductor ina channel formation region is used for a switching element whichcontrols supply of a power supply potential to a memory element.

In FIG. 13A, the structure of a memory device of this embodiment isillustrated as an example. The memory device illustrated in FIG. 13Aincludes a switching element 401 and a memory element group 403including a plurality of memory elements 402. Specifically, as each ofthe memory elements 402, a memory element having any of the structuresdescribed in Embodiments 1 to 5 can be used. Each of the memory elements402 included in the memory element group 403 is supplied with ahigh-level power supply potential VDD via the switching element 401.Further, each of the memory elements 402 included in the memory elementgroup 403 is supplied with a potential of a signal IN and a low-levelpower supply potential VSS.

In FIG. 13A, a transistor including an oxide semiconductor in a channelformation region is used for the switching element 401, and theswitching of the transistor is controlled by a signal Sig A supplied toa gate electrode thereof. Since the transistor used for the switchingelement 401 includes a highly-purified oxide semiconductor in a channelformation region, OFF-state current is extremely low as described above.

Note that in FIG. 13A, a structure in which the switching element 401includes only one transistor is illustrated; however, the presentinvention is not limited to this structure. In one embodiment of thepresent invention, the switching element 401 may include a plurality oftransistors. In the case where the plurality of transistors which serveas switching elements are included in the switching element 401, theplurality of transistors may be connected to each other in parallel, inseries, or in combination of parallel connection and series connection.

Although the switching element 401 controls the supply of the high-levelpower supply potential VDD to each of the memory elements 402 includedin the memory element group 403 in FIG. 13A, the switching element 401may control the supply of the low-level power supply potential VSS. InFIG. 13B, an example of a memory device in which each of the memoryelements 402 included in the memory element group 403 is supplied withthe low-level power supply potential VSS via the switching element 401is illustrated. The supply of the low-level power supply potential VSSto each of the memory elements 402 included in the memory element group403 can be controlled by the switching element 401.

Next, a structure of a transistor for a power device capable ofcontrolling higher voltage or higher current than the transistorincluding an oxide semiconductor in a channel formation regionillustrated in Embodiment 4 or Embodiment 5 will be described. By usinga transistor having such a structure for the switching element 401, thereliability of the memory device can be further increased. Note that thesame portions as Embodiment 4 or Embodiment 5 or portions havingfunctions similar to those of Embodiment 4 or Embodiment 5 can be formedas in Embodiment 4 or Embodiment 5, and also the same steps asEmbodiment 4 or Embodiment 5 or the steps similar to those of Embodiment4 or Embodiment 5 can be performed in a manner similar to those ofEmbodiment 4 or Embodiment 5; therefore, repetitive description thereofis omitted.

In FIG. 14A, a cross-sectional view of a transistor 420 described inthis embodiment is illustrated. FIG. 14B is a top view of the transistor420. A cross-sectional view along a dashed line B1-B2 in FIG. 14Bcorresponds to FIG. 14A.

The transistor 420 includes a first electrode 421 over an insulatingsurface.

The first electrode 421 is formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten,and yttrium; an alloy containing any of these metal elements as acomponent; an alloy containing these metal elements in combination; orthe like. Alternatively, one or more metal elements selected frommanganese, magnesium, zirconium, and beryllium can be used. In addition,the first electrode 421 can have a single-layer structure or a stackedstructure having two or more layers. For example, a single-layerstructure of an aluminum film containing silicon, a two-layer structurein which a titanium film is stacked over an aluminum film, a two-layerstructure in which a titanium film is stacked over a tungsten film, athree-layer structure in which a titanium film, an aluminum film, and atitanium film are stacked in the order, and the like can be given.Alternatively, a film, an alloy film, or a nitride film which containsaluminum and one or more elements selected from titanium, tantalum,tungsten, molybdenum, chromium, neodymium, and scandium may be used.

The first electrode 421 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. In addition, the first electrode 421 can have a stacked-layerstructure formed using the above light-transmitting conductive materialand the above metal element.

The first electrode 421 can be formed in such a manner that a conductivefilm is formed over the insulating surface by a sputtering method, a CVDmethod, or a vacuum evaporation method, a resist mask is formed over theconductive film in a photolithography process, and the conductive filmis etched using the resist mask. Alternatively, the first electrode 421is formed by a printing method or an inkjet method without using aphotolithography process, so that the number of steps can be reduced.Note that end portions of the first electrode 421 preferably have atapered shape, so that the coverage with a gate insulating film formedlater is improved. When the angle between the end portion of the firstelectrode 421 and the insulating surface over which the first electrode421 is formed is greater than or equal to 30° and less than or equal to60°, preferably greater than or equal to 40° and less than or equal to50°, the coverage with the gate insulating film formed later can beimproved.

In this embodiment, as the conductive film for forming the firstelectrode 421, a 50-nm-thick titanium film is formed by a sputteringmethod, a 100-nm-thick aluminum film is formed, and a 50-nm-thicktitanium film is formed. Next, etching is performed using the resistmask formed in the photolithography process, whereby the first electrode421 is formed. By forming a resist mask using an inkjet method insteadof the resist mask formed in the photolithography process, the number ofsteps can be reduced.

The transistor 420 includes an island-shaped oxide semiconductor film422 over the first electrode 421. The oxide semiconductor film 422 canbe formed by a sputtering method, a coating method, a printing method,or the like. In this embodiment, after an oxide semiconductor film isformed by a sputtering method over the first electrode 421, the oxidesemiconductor film is processed into a desired shape by etching or thelike, so that the island-shaped oxide semiconductor film 422 is formed.Moreover, the oxide semiconductor film can be formed by a sputteringmethod under a rare gas (for example, argon) atmosphere, an oxygenatmosphere, or an atmosphere including a rare gas (for example, argon)and oxygen.

Note that the etching for forming the island-shaped oxide semiconductorfilm 422 may be performed in accordance with description of the etchingof the oxide semiconductor film in Embodiment 4. Note that the angleformed between the end portion of the island-shaped oxide semiconductorfilm 422 which is formed by etching and the first electrode 421 ispreferably greater than or equal to 30° and less than or equal to 60°,more preferably greater than or equal to 40° and less than or equal to50°, whereby the coverage with the gate insulating film that is formedlater can be improved.

Note that before the oxide semiconductor film is formed by sputtering,dust on a surface of the first electrode 421 is preferably removed byreverse sputtering in which an argon gas is introduced and plasma isgenerated. The reverse sputtering means a method in which a surface of asubstrate is modified by application of voltage to the substrate side,not to a target side, in an argon atmosphere with the use of an RF powersource and by generating plasma in the vicinity of the substrate. Notethat instead of an argon atmosphere, a nitrogen atmosphere, a heliumatmosphere, or the like may be used. Alternatively, an argon atmosphereto which oxygen, nitrous oxide, or the like is added may be used.Alternatively, an argon atmosphere to which chlorine, carbontetrafluoride, or the like is added may be used.

For the oxide semiconductor film 422, the above-described oxidesemiconductor can be used.

In this embodiment, an In—Ga—Zn—O-based non-single-crystal film with athickness of 30 nm, which is obtained by a sputtering method using anoxide semiconductor target containing indium (In), gallium (Ga), andzinc (Zn), is used as the oxide semiconductor film 422. As the target, ametal oxide target containing each metal atom at a composition ratio ofIn:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used, forexample. In addition, the oxide semiconductor film can be formed by asputtering method in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or an atmosphere of a rare gas (typically, argon) andoxygen. In the case of using a sputtering method, a target containingSiO₂ at greater than or equal to 2 wt % and less than or equal to 10 wt% may be used for forming a film. The fill rate of the metal oxidetarget containing In, Ga, and Zn is higher than or equal to 90% andlower than or equal to 100%, preferably higher than or equal to 95% andlower than or equal to 99.9%. With the use of a metal oxide target withhigh fill rate, the oxide semiconductor film formed has high density.

The substrate is held in a treatment chamber kept under reducedpressure, a sputtering gas from which hydrogen and moisture are removedis introduced into the treatment chamber from which remaining moistureis being removed, and the oxide semiconductor film 422 is formed over asubstrate with the use of a metal oxide as a target. In film formation,the substrate temperature may be higher than or equal to 100° C. andlower than or equal to 600° C., preferably higher than or equal to 200°C. and lower than or equal to 400° C. Film formation is performed whilethe substrate is heated, whereby the impurity concentration in the oxidesemiconductor film formed can be reduced. In addition, damage bysputtering can be reduced. In order to remove moisture remaining in thetreatment chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the treatment chamber which is evacuated with thecryopump, for example, a hydrogen atom, a compound containing a hydrogenatom, such as water (H₂O), (more preferably, also a compound containinga carbon atom), and the like are removed, whereby the impurityconcentration in the oxide semiconductor film formed in the treatmentchamber can be reduced.

In this embodiment, as an example of a film formation condition of theoxide semiconductor film, the following conditions are applied: thesubstrate temperature is room temperature, the distance between thesubstrate and the target is 110 mm; the pressure is 0.4 Pa; the directcurrent (DC) power source is 0.5 kW; and the atmosphere contains oxygenand argon (oxygen flow rate of 15 sccm, argon flow rate of 30 sccm).Note that a pulse direct current (DC) power source is preferable becausedust called particles generated in film formation can be reduced and thefilm thickness can be uniform. The thickness of the oxide semiconductorfilm is greater than or equal to 1 μm, preferably greater than or equalto 3 μm, more preferably greater than or equal to 10 μm. Note that thepreferable thickness varies depending on an oxide semiconductor filmmaterial; thus, the appropriate thickness may be determined depending ona material.

Note that in order that hydrogen, hydroxyl group, and moisture arecontained as little as possible in the oxide semiconductor film 422, itis preferable that the substrate over which layers up to and includingthe first electrode 421 are formed be preheated in a preheating chamberof the sputtering apparatus, so that impurities such as hydrogen ormoisture absorbed onto the substrate is eliminated and evacuated, aspretreatment before film formation. The temperature for the preheatingis higher than or equal to 100° C. and less than or equal to 400° C.,preferably higher than or equal to 150° C. and less than or equal to300° C. As an evacuation unit provided in the preheating chamber, acryopump is preferable. Note that this preheating treatment can beomitted. Note also that this preheating may be similarly performed onthe substrate over which layers up to and including the gate electrodeare formed before the formation of the insulating film.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a deposition method using a sputtering method, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during film formationto form a thin compound film thereof, or a bias sputtering method inwhich voltage is also applied to a substrate during film formation canbe used.

Note that heat treatment is performed on the oxide semiconductor film422 in a reduced-pressure atmosphere, an atmosphere of an inert gas suchas nitrogen and a rare gas, an oxygen atmosphere, or an ultra-dry airatmosphere (the moisture amount is 20 ppm (−55° C. by conversion into adew point) or less, preferably 1 ppm or less, more preferably 10 ppb orless, in the case where measurement is performed by a dew point meter ina cavity ring down laser spectroscopy (CRDS) method). When the heattreatment is performed on the oxide semiconductor film 422, an oxidesemiconductor film 422 in which moisture or hydrogen is eliminated isformed. Specifically, heat treatment may be performed at higher than orequal to 300° C. and lower than or equal to 850° C. (or a temperaturelower than or equal to the strain point of a glass substrate). Sincedehydration or dehydrogenation can be performed in a short time with theRTA method, the heat treatment can be performed even at a temperatureover the strain point of a glass substrate. In this embodiment, heattreatment is performed on the oxide semiconductor film 422 at asubstrate temperature of 450° C. for 1 hour in a nitrogen atmospherewith the use of an electrical furnace that is one of heat treatmentapparatuses, and then, the oxide semiconductor film is not exposed toair, and water and hydrogen are prevented from entering. Impurities suchas moisture or hydrogen is eliminated by the heat treatment, so that theoxide semiconductor film 422 becomes an i-type (intrinsic) semiconductoror a substantially i-type semiconductor; therefore, deterioration ofcharacteristics of the transistor due to the impurities, such as shiftsin threshold voltage, can be prevented from being promoted and OFF-statecurrent can be reduced.

Detailed description of the heat treatment apparatus used for the heattreatment is already made in Embodiment 4 and therefore omitted here.

Note that it is preferable that in the heat treatment, moisture,hydrogen, or the like be not contained in nitrogen or a rare gas such ashelium, neon, or argon. It is preferable that the purity of nitrogen orthe rare gas such as helium, neon, or argon which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

When impurities are added to an oxide semiconductor, in a gatebias-temperature stress test (BT test, the test condition is for exampleat 85° C. with 2×10⁶ V/cm for 12 hours), a bond between the impuritiesand a main component of the oxide semiconductor is broken by a highelectrical field (B: bias) and high temperature (T: temperature), and adangling bond generated causes drift of the threshold voltage (Vth).However, in the above-described manner, by improving the interfacecharacteristics between the gate insulating film and the oxidesemiconductor film and removing impurities, particularly hydrogen,water, and the like, in the oxide semiconductor film as much aspossible, a transistor which remains stable even with respect to the BTtest can be obtained.

Through the above-described steps, the hydrogen concentration in theoxide semiconductor film can be reduced and the oxide semiconductor filmcan be highly purified. Thus, the oxide semiconductor film can bestabilized. In addition, heat treatment at a temperature of lower thanor equal to the glass transition temperature makes it possible to forman oxide semiconductor film with a wide band gap in which carrierdensity is extremely low. Therefore, a transistor can be manufacturedusing a large-sized substrate, so that productivity can be increased. Inaddition, by using the oxide semiconductor film in which the hydrogenconcentration is reduced and purity is improved, it is possible tomanufacture a transistor with high withstand voltage, a reducedshort-channel effect, and a high on-off ratio.

The transistor 420 also includes a second electrode 423 over the oxidesemiconductor film 422. The material and structure of the conductivefilm used for the second electrode 423 can be similar to those for thefirst electrode 421. In addition, a method for forming the secondelectrode 423 can be similar to that for the first electrode 421.

In this embodiment, a resist mask is formed over the conductive filmwhich serves as the second electrode 423 in a photolithography process,and the conductive film is etched using the resist mask, so that thesecond electrode 423 is formed. Here, as the conductive film for formingthe second electrode 423, a 50-nm-thick titanium film, a 100-nm-thickaluminum film, and a 50-nm-thick titanium film are stacked in thatorder. When the angle formed between end portion of the second electrode423 and the oxide semiconductor film 422 is preferably greater than orequal to 30° and less than or equal to 60°, more preferably greater thanor equal to 40° and less than or equal to 50°, the coverage with a gateinsulating film formed later can be improved. In addition, the secondelectrode 423 is formed away from the first electrode 421 so as not tobe in contact with the first electrode 421.

One of the first electrode 421 and the second electrode 423 functions asa source electrode of a transistor, and the other functions as a drainelectrode thereof.

Heat treatment may be performed after the formation of the secondelectrode 423. The temperature of the heat treatment is higher than orequal to 400° C. and lower than or equal to 850° C., preferably higherthan or equal to 400° C. and lower than the strain point of thesubstrate. In this embodiment, the substrate is introduced into anelectric furnace which is one of heat treatment apparatuses, and heattreatment is performed on the oxide semiconductor film 422 in an inertgas atmosphere, such as a nitrogen atmosphere or a rare gas atmosphere,at 450° C. for 1 hour, and then the oxide semiconductor film is notexposed to air. Accordingly, hydrogen, water, hydroxyl group, hydride,or the like can be prevented from being mixed into the oxidesemiconductor film, hydrogen concentration is further reduced, and theoxide semiconductor film is highly purified, whereby an i-type oxidesemiconductor film or a substantially i-type oxide semiconductor filmcan be obtained.

Note that it is preferable that in the heat treatment, hydrogen, water,hydroxyl group, hydride, or the like be not contained in nitrogen or arare gas such as helium, neon, or argon. Alternatively, the purity ofnitrogen or a rare gas such as helium, neon, or argon introduced into aheat treatment apparatus is preferably 6N (99.9999%) or higher, morepreferably 7N (99.99999%) or higher (that is, the concentration of theimpurities is 1 ppm or lower, preferably 0.1 ppm or lower).

The transistor 420 also includes a gate insulating film 424 covering thefirst electrode 421, the oxide semiconductor film 422, and the secondelectrode 423, and a gate electrode 425 which is formed over the gateinsulating film 424. The gate insulating film 424 is formed using a filmhaving a single layer or a stacked layer which includes one or more of asilicon oxide film, a silicon nitride film, a silicon oxynitride film, asilicon nitride oxide film, an aluminum oxide film, an aluminum nitridefilm, an aluminum oxynitride film, an aluminum nitride oxide film, ahafnium oxide film, and a tantalum oxide film by plasma CVD, sputtering,or the like.

When the gate insulating film 424 is formed using a high-k material suchas hafnium silicate (HfSiOx), HfSixOy to which N is added, hafniumaluminate (HfAlOx) to which N is added, hafnium oxide, or yttrium oxide,gate leakage current can be reduced. Further, a stacked structure inwhich a high-k material and one or more of a silicon oxide film, asilicon nitride film, a silicon oxynitride film, a silicon nitride oxidefilm, and an aluminum oxide film are stacked can be used. The thicknessof the gate insulating film 424 is preferably greater than or equal to50 nm and less than or equal to 500 nm. When the thickness of the gateinsulating film 424 is large, the gate leakage current can be reduced.

It is preferable that the gate insulating film 424 contain impuritiessuch as moisture or hydrogen as little as possible. In the case where asilicon oxide film is formed by a sputtering method, a silicon target ora quartz target is used as a target and a mixed gas of oxygen and argonis used as a sputtering gas.

The oxide semiconductor that is made to be an intrinsic oxidesemiconductor or a substantially intrinsic oxide semiconductor (theoxide semiconductor that is highly purified) by removal of impurities isextremely sensitive to an interface state and an interface electriccharge; thus, an interface between the oxide semiconductor and the gateinsulating film 424 is important. Therefore, the gate insulating film(GI) that is in contact with the highly-purified oxide semiconductorneeds to have a higher quality.

For example, high-density plasma CVD using microwaves (2.45 GHz) ispreferable because a dense high-quality insulating film having highwithstand voltage can be formed. This is because when thehighly-purified oxide semiconductor is closely in contact with thehigh-quality gate insulating film, the interface state can be reducedand interface properties can be favorable.

Needless to say, another method such as sputtering method or plasma CVDmethod can be employed as long as the method enables formation of agood-quality insulating film as a gate insulating film 424. In addition,any insulating film can be used as long as film quality and interfaceproperties with an oxide semiconductor of the gate insulating film 424are modified by heat treatment performed after deposition. In any case,any insulating film can be used as long as film quality as a gateinsulating film is high, interface state density with an oxidesemiconductor is decreased, and a favorable interface can be formed.

The gate insulating film 424 may have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film having a lower proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film are stacked. In this case, theinsulating film such as a silicon oxide film or a silicon oxynitridefilm is formed between the insulating film having a barrier property andthe oxide semiconductor film. As the insulating film having a highbarrier property, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, an aluminum nitride oxide film, or the likecan be given, for example. The insulating film having a barrier propertyis used, so that impurities in an atmosphere, such as moisture orhydrogen, or impurities contained in the substrate, such as an alkalimetal or a heavy metal, can be prevented from entering the oxidesemiconductor film, the gate insulating film 424, or at the interfacebetween the oxide semiconductor film and another insulating film and thevicinity thereof. In addition, the insulating film having a lowerproportion of nitrogen such as a silicon oxide film or a siliconoxynitride film is formed so as to be in contact with the oxidesemiconductor film, so that the insulating film formed using a materialhaving a high barrier property can be prevented from being in directcontact with the oxide semiconductor film.

For example, a gate insulating film having a total thickness of 100 nmmay be formed in such a manner that a silicon oxide film (SiO_(x) (x>0))with a thickness of greater than or equal to 5 nm and less than or equalto 300 nm is formed as a first gate insulating film and a siliconnitride film (SiN_(y) (y>0)) with a thickness of greater than or equalto 50 nm and less than or equal to 200 nm is stacked over the first gateinsulating film as a second gate insulating film by sputtering. In thisembodiment, a 100-nm-thick silicon oxide film is formed by an RFsputtering method under a condition that the pressure is 0.4 Pa, thehigh-frequency power is 1.5 kW, and an atmosphere containing oxygen andargon (an oxygen flow rate of 25 sccm: an argon flow rate of 25sccm=1:1) is used.

Note that in order that hydrogen, hydroxyl group, and moisture arecontained as little as possible in the gate insulating film 424, it ispreferable that the substrate over which first electrode 421, the oxidesemiconductor film 422 and the second electrode 423 are formed bepreheated in a preheating chamber of the sputtering apparatus, so thatimpurities such as hydrogen or moisture absorbed onto the substrate iseliminated and evacuated, as pretreatment before film formation. Thetemperature for the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably higher than or equal to 150°C. and lower than or equal to 300° C. As an evacuation unit provided inthe preheating chamber, a cryopump is preferable. Note that thispreheating treatment can be omitted.

Note that after the gate insulating film 424 is formed, heat treatmentmay be performed. The heat treatment is performed in an air atmosphereor an inert gas atmosphere (nitrogen, helium, neon, argon, or the like)at higher than or equal to 200° C. and lower than or equal to 400° C.,for example, at higher than or equal to 250° C. and lower than or equalto 350° C. In this embodiment, for example, heat treatment for 1 hour at250° C. in a nitrogen atmosphere is performed. By the heat treatmentperformed in the state where silicon oxide contained in the gateinsulating film 424 is in contact with the oxide semiconductor film 422,oxygen is supplied from the silicon oxide even when an oxygen deficiencyis generated in the above-described heat treatment for eliminatingmoisture or hydrogen, whereby the oxygen deficiency which forms donorscan be reduced, a structure which satisfies the stoichiometriccomposition ratio can be obtained, and the oxide semiconductor film 422is made to be an i-type semiconductor film or a substantially i-typesemiconductor film. There is no particular limitation on the timing ofthis heat treatment as long as it is after the formation of the gateinsulating film 424. The heat treatment may be performed after anotherstep, for example, after any one of the gate electrode 425, aninsulating film 426, and wirings 434, 435, and 436 is formed. This heattreatment can be performed without increasing the number ofmanufacturing steps by doubling as another step such as heat treatmentfor reduction of the resistance of a transparent conductive film.

The gate electrode 425 can be formed using a single layer or a stackedlayer of one or more conductive films including a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, neodymium, orscandium, or an alloy material which includes any of these metalmaterials as a main component; or a nitride of any of these metals. Notethat aluminum or copper can also be used as such metal materials as longas aluminum or copper can withstand a temperature of heat treatmentperformed in a later process. Aluminum or copper is preferably combinedwith a refractory metal material so as to prevent a heat resistanceproblem and a corrosive problem. As the refractory metal material,molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium,or the like can be used.

For example, as a two-layer structure of the gate electrode 425, thefollowing structures are preferable: a two-layer structure in which amolybdenum film is stacked over an aluminum film, a two-layer structurein which a molybdenum film is stacked over a copper film, a two-layerstructure in which a titanium nitride film or a tantalum nitride film isstacked over a copper film, and a two-layer structure in which atitanium nitride film and a molybdenum film are stacked. As athree-layer structure of the gate electrode 425, the following structureis preferable: a stacked structure containing an aluminum film, an alloyfilm of aluminum and silicon, an alloy film of aluminum and titanium, oran alloy film of aluminum and neodymium in a middle layer and any of atungsten film, a tungsten nitride film, a titanium nitride film, and atitanium film in a top layer and a bottom layer.

Further, by using a light-transmitting oxide conductive film of indiumoxide, indium tin oxide, an indium oxide-zinc oxide alloy, zinc oxide,zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, orthe like for the gate electrode 425, the aperture ratio of a pixelportion can be increased.

The gate electrode 425 is formed to a thickness of 10 nm to 400 nm,preferably 100 nm to 200 nm. In this embodiment, after a conductive filmwith a thickness of 150 nm for the gate electrode is formed by asputtering method using a tungsten target, the conductive film isprocessed (patterned) by etching to have a desired shape, so that thegate electrode 425 is formed. The gate electrode 425 is formed at leastso as to overlap with the end portion of the oxide semiconductor film422 with the gate insulating film 424 provided therebetween. In the endportion of the oxide semiconductor film 422, a channel formation regionis formed in a portion which overlaps with the gate electrode 425 withthe gate insulating film 424 provided therebetween. Note that the gateelectrode 425 preferably has a tapered shape because coverage with theinsulating film 426 stacked thereover can be improved. Note that aresist mask may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask; thus, manufacturing costcan be reduced.

The transistor 420 also includes the insulating film 426 covering thefirst electrode 421, the oxide semiconductor film 422, the secondelectrode 423, the gate insulating film 424, and the gate electrode 425.The insulating film 426 preferably contains impurities such as moistureor hydrogen as little as possible, and may be formed using asingle-layer insulating film or a plurality of insulating films stacked.The insulating film 426 is formed using, for example, an oxideinsulating film such as a silicon oxide film, a silicon oxynitride film,an aluminum oxide film, or an aluminum oxynitride film; or a nitrideinsulating film such as a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, or an aluminum nitride oxide film.Alternatively, an oxide insulating film and a nitride insulating filmcan be stacked. An insulating film having a high barrier property, forexample, a silicon nitride film, a silicon nitride oxide film, analuminum nitride film, or an aluminum nitride oxide film may be used forthe insulating film 426, so that impurities such as moisture or hydrogencan be prevented from entering the oxide semiconductor film 422, thegate insulating film 424, or the interface between the oxidesemiconductor film 422 and another insulating film and the vicinitythereof.

In this embodiment, the insulating film 426 is formed to have astructure in which a 100-nm-thick silicon nitride film formed by asputtering method is stacked over a 200-nm-thick silicon oxide filmformed by a sputtering method. Note that when the insulating film 426 isformed by a sputtering method, the substrate is heated to a temperatureof 100° C. to 400° C., a sputtering gas in which hydrogen, water,hydroxyl group, hydride, or the like is removed and which containshigh-purity nitrogen is introduced, and the insulating film 426 may beformed using a silicon semiconductor target. Also in this case, aninsulating film is preferably formed in the state where hydrogen, water,hydroxyl group, hydride, or the like remaining in the treatment chamberis removed.

Note that after the insulating film 426 is formed, heat treatment may beperformed. The heat treatment is performed in an inert gas atmosphere(nitrogen, helium, neon, argon, or the like) at higher than or equal to200° C. and lower than or equal to 400° C., for example, at higher thanor equal to 250° C. and lower than or equal to 350° C.

Contact holes 431, 432, and 433 are formed as follows: a resist mask isformed by a photolithography process and parts of the gate insulatingfilm 424 and the insulating film 426 are selectively etched using theresist mask. A part of the gate electrode 425 is exposed in the contacthole 431; a part of the second electrode 423 is exposed in the contacthole 432; and a part of the gate electrode 425 is exposed in the contacthole 433. At the time of the formation of these contact holes, a contacthole so as to expose the first electrode 421 may be formed in a regionof the first electrode 421, which is not covered with the gate electrode425.

The wiring 434, the wiring 435, and the wiring 436 are connected to thegate electrode 425, the second electrode 423, and the gate electrode 425through the contact hole 431, the contact hole 432, and the contact hole433 which are formed in the insulating film 426, respectively. Note thatin formation of these wirings, a wiring connected to the first electrode421 through a contact hole may be formed.

The wirings 434, 435, and 436 can be formed using a conductive filmhaving a structure and a material which are similar to those for thefirst electrode 421 by a manufacturing method similar to that for thefirst electrode 421.

In FIG. 14B, a wiring 440 is a wiring formed at the same time as thewirings 434, 435, and 436 and is connected to the first electrode 421through a contact hole 441.

Through the above-described steps, the hydrogen concentration in theoxide semiconductor film can be reduced and the oxide semiconductor filmcan be highly purified. Thus, the oxide semiconductor film can bestabilized. In addition, heat treatment at a temperature of lower thanor equal to the glass transition temperature makes it possible to forman oxide semiconductor film with a wide band gap in which carrierdensity is extremely low. Therefore, a transistor can be manufacturedusing a large-sized substrate, so that productivity can be increased. Inaddition, by using the oxide semiconductor film in which the hydrogenconcentration is reduced and purity is improved, it is possible tomanufacture a transistor with high withstand voltage, a reducedshort-channel effect, and a high on-off ratio.

Although all the part of the oxide semiconductor film 422, which isformed in a region which is different from the region where the secondelectrode 423 is formed is covered with the gate electrode 425 in thisembodiment, the present invention is not limited to this structure aslong as at least a part of the portion of the oxide semiconductor film422, which is formed in a region different from the region where thesecond electrode 423 is formed, is covered with the gate electrode 425.

Here, the drain withstand voltage of the transistor described in thisembodiment is described below.

When the electric field in the semiconductor reaches a certain thresholdvalue, impact ionization occurs, carriers accelerated by the highelectric field impact crystal lattices in a depletion layer, therebygenerating pairs of electrons and holes. When the electric field becomeseven higher, the pairs of electrons and holes generated by the impactionization are further accelerated by the electric field, and the impactionization is repeated, resulting in an avalanche breakdown in whichcurrent is increased exponentially. The impact ionization occurs becausecarriers (electrons and holes) have kinetic energy that is larger thanor equal to the band gap of the semiconductor. Therefore, as the bandgap is larger, the electric field needed to cause the impact ionizationis higher.

Since the band gap of the oxide semiconductor is 3.15 eV, which islarger than the band gap of amorphous silicon, 1.74 eV, the avalanchebreakdown is expected to be unlikely to occur. Therefore, a transistorincluding the oxide semiconductor has a high drain withstand voltage,and an exponential sudden increase of ON-state current is expected to beunlikely to occur when a high electric field is applied.

Next, hot-carrier degradation of a transistor including an oxidesemiconductor will be described.

The hot-carrier degradation means deterioration of transistorcharacteristics, e.g., variation in the threshold voltage or leakagecurrent, which is caused as follows: electrons that are accelerated tobe rapid are injected into a gate insulating film in the vicinity of adrain in a channel and become fixed electric charge or form trap levelsat the interface between the gate insulating film and the oxidesemiconductor. The factors of the hot-carrier degradation are, forexample, channel-hot-electron injection (CHE injection) anddrain-avalanche-hot-carrier injection (DAHC injection).

Since the band gap of silicon is narrow, electrons are likely to begenerated like an avalanche owing to an avalanche breakdown, andelectrons that are accelerated to be so rapid as to go over a barrier tothe gate insulating film are increased in number. However, the oxidesemiconductor described in this embodiment has a wide band gap;therefore, the avalanche breakdown is unlikely to occur and resistanceto the hot-carrier degradation is higher than that of silicon. Note thatalthough the band gap of silicon carbide which is one of materialshaving high withstand voltage and that of an oxide semiconductor aresubstantially equal to each other, electrons are less likely to beaccelerated, hot-carrier degradation is less likely to be caused than inthe case of silicon carbide, and drain withstand voltage is high in theoxide semiconductor because the mobility of the oxide semiconductor islower than that of silicon carbide by approximately two orders ofmagnitude.

From the above, a transistor including an oxide semiconductor has highdrain withstand voltage; specifically, such a transistor can have adrain withstand voltage of greater than or equal to 100 V, preferablygreater than or equal to 500 V, more preferably greater than or equal to1 kV.

Comparison between a transistor using silicon carbide, which is atypical example of a transistor, and a transistor using an oxidesemiconductor will be described below. Here, 4H—SiC is used as thesilicon carbide.

An oxide semiconductor and 4H—SiC have some things in common. Oneexample is intrinsic carrier density. Using the Fermi-Dirac distributionat normal temperature, the intrinsic carrier density of the oxidesemiconductor is estimated to approximately 10⁻⁷ cm⁻³, which isextremely low like the carrier density of 4H—SiC, i.e., 6.7×10⁻¹¹ cm⁻³.

In addition, the energy band gap of the oxide semiconductor is 3.0 eV to3.5 eV and that of 4H—SiC is 3.26 eV, which means that both the oxidesemiconductor and the silicon carbide are wide-gap semiconductors.

However, the manufacturing temperature of a transistor including anoxide semiconductor and the manufacturing temperature of a transistorincluding silicon carbide are quite different. Silicon carbide generallyneeds heat treatment at 1500° C. to 2000° C. In contrast, an oxidesemiconductor can be formed by heat treatment at 300° C. to 850° C.,which allows a transistor to be manufactured over a large-sizedsubstrate. In addition, throughput can be improved.

Further, a manufacturing process of the transistor including siliconcarbide, which uses a PN junction, involves a step of doping with animpurity element that can be a donor or an acceptor (e.g., phosphorus orboron); therefore, the number of manufacturing steps is increased. Onthe other hand, the transistor including an oxide semiconductor is notneeded to be provided with a PN junction; therefore, the number ofmanufacturing steps can be decreased and the throughput can be improved,and further, a large-sized substrate can be used.

Note that considerable research has been done on properties of oxidesemiconductors such as density of states (DOS) in the band gap; however,the research does not include the idea of sufficiently reducing the DOSitself. In this embodiment, a highly-purified oxide semiconductor isformed by removing water or hydrogen which might induce the DOS from theoxide semiconductor. This is based on the idea of sufficiently reducingthe DOS itself. Such a highly-purified oxide semiconductor enablesfabrication of very excellent industrial products.

Further, it is also possible to form a more highly-purified (i-type)oxide semiconductor by supplying oxygen to a dangling bond of metalwhich is generated by oxygen vacancy and reducing the DOS due to theoxygen defects. For example, an oxide film containing excessive oxygenis formed in close contact with a channel formation region and thenoxygen is supplied to the channel formation region from the oxide film,so that the DOS due to oxygen defects can be reduced.

It can be considered that a factor of a defect of an oxide semiconductoris a shallow level due to excessive hydrogen at 0.1 eV to 0.2 eV belowthe conduction band, a deep level resulted from lack of oxygen, or thelike. Thorough removal of hydrogen and sufficient supply of oxygen forelimination of such a defect would be right as a technical thought.

An oxide semiconductor is generally considered as an n-typesemiconductor; however, in this embodiment, an i-type oxidesemiconductor is realized by removing impurities, particularly water orhydrogen. In this point, the technical thought of the present inventionis novel as compared with a conventional technical thought in which ani-type semiconductor is formed by adding impurities to a semiconductorsuch as silicon.

By making the oxide semiconductor be an i-type oxide semiconductor,favorable temperature characteristics of the transistor can be obtained;specifically, in terms of the current vs. voltage characteristics of thetransistor, ON-state current, OFF-state current, field-effect mobility,an S value, and a threshold voltage are hardly fluctuated at atemperature ranging from −25° C. to 150° C., and the current vs. voltagecharacteristics are hardly degraded by the temperature.

In the transistor using an oxide semiconductor which is described inthis embodiment, mobility is lower than that in a transistor usingsilicon carbide by approximately two orders of magnitude; however,current value of the transistor and device characteristics can beimproved by increasing the drain voltage and the channel width (W).

A technical idea of this embodiment is that a substance is not added toan oxide semiconductor and on the contrary the oxide semiconductoritself is highly-purified by intentionally removing impurities such aswater or hydrogen which undesirably exist therein. In other words, theoxide semiconductor is highly-purified by removing water or hydrogenwhich forms a donor level, reducing oxygen deficiency, and sufficientlysupplying oxygen that is a component of the oxide semiconductor.

At the time of deposition, the oxide semiconductor includes hydrogen atapproximately 10²⁰ cm⁻³ which is measured by secondary ion massspectrometry (SIMS). The oxide semiconductor is highly purified and madeto be an electrically i-type (intrinsic) semiconductor by intentionallyremoving water or hydrogen which forms a donor level and further byadding oxygen (one of components of the oxide semiconductor), which isreduced at the same time as removal of water or hydrogen, to the oxidesemiconductor.

In this embodiment, the amount of water and hydrogen in the oxidesemiconductor is preferably as small as possible, and the number ofcarriers in the oxide semiconductor is preferably as small as possible.Specifically, a carrier density of lower than 1×10¹⁴ cm³, preferablylower than 1×10¹² cm⁻³, more preferably lower than 1×10¹¹ cm⁻³ which islower than or equal to the measurement limit is desirable. The carriersin the oxide semiconductor are reduced or preferably eliminated, inother words, the oxide semiconductor is made to be an i-type (intrinsic)semiconductor, so that the oxide semiconductor functions as a path in atransistor through which carriers pass. As a result, OFF-state currentcan be extremely small when the transistor is off. The above is thetechnical idea of this embodiment.

In addition, the oxide semiconductor functions as a path of carriers,and the oxide semiconductor itself is an i-type (intrinsic)semiconductor which is highly-purified so as to include no carriers orextremely few carriers, and thus carriers are supplied by a sourceelectrode and a drain electrode.

The transistor having the structure described in this embodiment canless occupy a substrate surface than a horizontal transistor in which achannel is formed substantially in parallel with a substrate asdescribed in Embodiment 4. As a result, it is possible to miniaturizethe transistor.

As described above, the oxide semiconductor film is highly purified sothat impurities other than a main component of the oxide semiconductorfilm, typically hydrogen, water, hydroxyl group, or hydride, may becontained as little as possible, whereby good operation of thetransistor can be obtained. In particular, withstand voltage can behigher, a short channel effect can be reduced, and a high on-off ratiocan be realized.

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

Embodiment 7

In this embodiment, a method for forming an oxide semiconductor film,which is different from that in Embodiment 4, is described withreference to FIGS. 15A to 15E.

First, a gate electrode 701 and a gate electrode 702 are formed over aninsulating surface, and then a gate insulating film 703 is formed overthe gate electrode 701 and the gate electrode 702 (see FIG. 15A). Sincematerials, structures, and thicknesses of the gate electrode 701, thegate electrode 702, and the gate insulating film 703 are alreadydescribed in Embodiment 4, the detailed description thereof is omittedin this embodiment.

Next, as illustrated in FIG. 15A, a first oxide semiconductor film 730with a thickness of greater than or equal to 2 nm and less than or equalto 15 nm is formed over the gate insulating film 703. The first oxidesemiconductor film 730 can be formed by a sputtering method in a raregas (typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere including a rare gas (for example, argon) and oxygen.

Note that before the first oxide semiconductor film 730 is formed by asputtering method, dust on a surface of the gate insulating film 703 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering means amethod in which a surface of a substrate is modified by application ofvoltage to the substrate side, not to a target side, in an argonatmosphere with the use of an RF power source and by generating plasmain the vicinity of the substrate. Note that instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, or the like maybe used. Alternatively, an argon atmosphere to which oxygen, nitrousoxide, or the like is added may be used. Alternatively, an argonatmosphere to which chlorine, carbon tetrafluoride, or the like is addedmay be used.

For the first oxide semiconductor film 730, the above-described oxidesemiconductor can be used.

In this embodiment, as the first oxide semiconductor film 730, anIn—Ga—Zn—O-based non-single-crystal film with a thickness of 5 nm, whichis obtained by a sputtering method using a metal oxide target includingindium (In), gallium (Ga), and zinc (Zn), is used. As the target, forexample, a metal oxide target having a composition ratio of metal atoms,In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. In thisembodiment, a metal oxide target which tends to be crystallized ispreferably used in order to intentionally perform crystallization byheat treatment performed later. The fill rate of the metal oxide targetcontaining In, Ga, and Zn is higher than or equal to 90% and lower thanor equal to 100%, and preferably higher than or equal to 95% and lowerthan or equal to 99.9%. When a metal oxide target having a high fillrate is used, the impurity concentration in an oxide semiconductor filmto be formed can be reduced, so that a transistor with excellentelectric characteristics or high reliability can be obtained.

The substrate is held in a treatment chamber kept under reducedpressure, a sputtering gas from which hydrogen and moisture are removedis introduced into the treatment chamber from which remaining moistureis being removed, and the first oxide semiconductor film 730 is formedover the insulating surface with the use of a metal oxide as a target.In film formation, the substrate temperature may be higher than or equalto 100° C. and lower than or equal to 600° C., preferably higher than orequal to 200° C. and lower than or equal to 400° C. Film formation isperformed while the substrate is heated, whereby the impurityconcentration in the oxide semiconductor film formed can be reduced. Inaddition, damage by sputtering can be reduced. In order to removemoisture remaining in the treatment chamber, an entrapment vacuum pumpis preferably used. For example, a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. The evacuation unit may be a turbopump provided with a cold trap. In the treatment chamber which isevacuated with the cryopump, for example, a hydrogen atom, a compoundcontaining a hydrogen atom, such as water (H₂O), (more preferably, alsoa compound containing a carbon atom), and the like are removed, wherebythe impurity concentration in the oxide semiconductor film formed in thetreatment chamber can be reduced.

An example of the deposition condition is as follows: the distancebetween the substrate and the target is 170 mm, the pressure is 0.4 Pa,the direct current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the flow rate of oxygen is 100%). Note that a pulsedirect current (DC) power source is preferable because dust calledparticles generated in film formation can be reduced and the filmthickness can be uniform. The thickness of the oxide semiconductor filmis preferably greater than or equal to 5 nm and less than or equal to 30nm. Since appropriate thickness depends on an oxide semiconductormaterial used, the thickness can be determined as appropriate dependingon the material.

Note that in order that hydrogen, hydroxyl group, and moisture arecontained as little as possible in the first oxide semiconductor film730, it is preferable that the substrate over which layers up to andincluding the gate insulating film 703 are formed be preheated in apreheating chamber of the sputtering apparatus, so that impurities suchas hydrogen or moisture absorbed onto the substrate is eliminated andevacuated, as pretreatment before film formation. The temperature forthe preheating is higher than or equal to 100° C. and lower than orequal to 600° C., preferably higher than or equal to 150° C. and lowerthan or equal to 300° C. As an evacuation unit provided for thepreheating chamber, a cryopump is preferable. Note that this preheatingtreatment can be omitted.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a deposition method using a sputtering method, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, or a bias sputtering method in whichvoltage is also applied to a substrate during deposition can be used.

The gate insulating film 703 and the first oxide semiconductor film 730may be formed successively without exposure to air. Successive filmformation without exposure to air makes it possible to obtain eachinterface between stacked layers, which is not contaminated byatmospheric components or impurity elements floating in air, such aswater, hydrocarbon, or the like. Therefore, variation in characteristicsof the transistor can be reduced.

Next, first heat treatment is performed and crystals are grown from asurface of the first oxide semiconductor film 730 as illustrated in FIG.15B, so that a first oxide semiconductor film 731 in which at least apart thereof is crystallized or which becomes single crystals isobtained. The first heat treatment is performed at higher than or equalto 450° C. and lower than or equal to 850° C., preferably higher than orequal to 600° C. and lower than or equal to 700° C. Heating time isgreater than or equal to 1 minute and less than or equal to 24 hours.The single crystal layer grows from the surface to the inside portionand contains plate-shaped crystals whose average thickness is greaterthan or equal to 2 nm and less than or equal to 10 nm. The crystal layerformed at the surface has the a-b plane and is c-axis oriented in adirection perpendicular to the surface. In this embodiment, an examplein which the entire first oxide semiconductor film 731 is crystallized(the crystals are also referred to as co-growing (CG) crystals) by thefirst heat treatment is described.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen, oxygen, or a raregas such as helium, neon, or argon. In addition, nitrogen, oxygen, or arare gas such as helium, neon, or argon which is introduced into a heattreatment apparatus preferably has a purity of 6N (99.9999%) or higher,more preferably 7N (99.99999%) or higher (that is, the impurityconcentration is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm). Further, the first heat treatment may be performed ina dry air atmosphere with an H₂O concentration of lower than or equal to20 ppm.

In this embodiment, heat treatment in a dry air atmosphere at 700° C.for 1 hour is performed as the first heat treatment.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may be provided with a device for heating an object to beprocessed by heat conduction or heat radiation from a heating elementsuch as a resistance heating element. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react withan object to be processed by heat treatment, such as nitrogen or a raregas such as argon is used.

For example, the heat treatment can employ GRTA, in which the substrateis moved into an inert gas heated at a high temperature of 650° C. to700° C., and heated for several minutes there, and then the substrate ismoved out of the high-temperature inert gas. With GRTA, high-temperatureheat treatment for a short period of time can be achieved.

Next, as illustrated in FIG. 15C, a second oxide semiconductor film 732is formed over the first oxide semiconductor film 731, which includesplate-shaped single crystals. The thickness of the second oxidesemiconductor film 732 is at least larger than that of the first oxidesemiconductor film 731 and less than or equal to 10 μm. Note that anappropriate thickness of the second oxide semiconductor film 732 may bedetermined by a practitioner depending on a device manufactured. Forexample, in the case where a bottom-gate transistor is manufactured, thetotal thickness of the first oxide semiconductor film 731 and the secondoxide semiconductor film 732 is greater than or equal to 10 nm and lessthan or equal to 200 nm. In addition, for example, in the case where atransistor is manufactured, the total thickness of the first oxidesemiconductor film 731 and the second oxide semiconductor film 732 isgreater than or equal to 10 nm and less than or equal to 50 nm. Thesecond oxide semiconductor film 732 can be formed by a sputtering methodin a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically, argon) and oxygen.

The above oxide semiconductor can be used for the second oxidesemiconductor film 732.

It is preferable that the first oxide semiconductor film 731 and thesecond oxide semiconductor film 732 be formed using materials containingthe same components, or have the same crystal structure and closelattice constants (lattice mismatch is less than or equal to 1%). In thecase where the materials containing the same components are used,crystal growth is further promoted from the single crystal layer of thefirst oxide semiconductor film 731 in the crystallization performedlater. In addition, when the materials contain the same components,physical properties of an interface such as adhesion or electriccharacteristics are favorable. Note that the second oxide semiconductorfilm 732 is preferably formed using a material whose electriccharacteristics (e.g., mobility, threshold voltage, or band gap) isimproved more than the first oxide semiconductor film 731 whencrystallinity is increased.

Next, second heat treatment is performed, so that crystals are grownfrom the first oxide semiconductor film 731 toward the second oxidesemiconductor film 732. The second heat treatment is performed at higherthan or equal to 450° C. and lower than or equal to 850° C., preferablyhigher than or equal to 550° C. and lower than or equal to 650° C.Heating time is greater than or equal to 1 minute and less than or equalto 24 hours. By the second heat treatment, as illustrated in FIG. 15D,an oxide semiconductor film 733 including the crystallized first oxidesemiconductor film 731 and a crystallized second oxide semiconductorfilm 735 can be obtained.

In the case where oxide semiconductor materials used for the first oxidesemiconductor film 731 and the second oxide semiconductor film 735contain the same components, the first oxide semiconductor film 731 andthe second oxide semiconductor film 735 have the same crystal structure.The second oxide semiconductor film 735 is formed through axial growthor epitaxial growth from the first oxide semiconductor film 731, andthus, the c-axes of the second oxide semiconductor film 735 and thefirst oxide semiconductor film 731 are identical with each other. As aresult, in the oxide semiconductor film 733, actually, the boundarybetween the first oxide semiconductor film 731 and the second oxidesemiconductor film 735 is unclear.

In some cases, the oxide semiconductor film 733 includes polycrystals ina region overlapping with an uneven part of the gate insulating film,thereby including a crystal boundary. In addition, a region of the oxidesemiconductor film 733 which serves as a channel formation regionoverlaps with at least a planar part of the gate insulating film, andtherefore there is a case where the first oxide semiconductor film 731and the second oxide semiconductor film 735 include c-axis-orientedsingle crystals. In the case where the first oxide semiconductor film731 and the second oxide semiconductor film 735 are c-axis-oriented, thefirst oxide semiconductor film 731 and the second oxide semiconductorfilm 735 desirably have identical a-b planes, a-axes, or b-axes;however, the direction of the a-axes or b-axes may be different in somecases.

Note that also in the second heat treatment, it is preferable thatwater, hydrogen, and the like be not contained in nitrogen, oxygen, or arare gas such as helium, neon, or argon. Alternatively, nitrogen,oxygen, or a rare gas such as helium, neon, or argon which is introducedinto a heat treatment apparatus preferably has a purity of 6N or higher,more preferably 7N or higher (that is, the impurity concentration islower than or equal to 1 ppm, preferably lower than or equal to 0.1ppm). Further, the second heat treatment may be performed in anultra-dry air atmosphere with an H₂O concentration of 20 ppm or lower.In addition, at the time of increasing the temperature in the secondheat treatment, an atmosphere of a furnace may be a nitrogen atmosphereand the atmosphere may be switched to an oxygen atmosphere at the timeof performing cooling.

Note that the heat treatment apparatus for the second heat treatment isnot limited to a particular apparatus, and the apparatus may be providedwith a device for heating an object to be processed by heat radiation orheat conduction from a heating element such as a resistance heatingelement. For example, an electric furnace, or an RTA apparatus such as aGRTA apparatus or an LRTA apparatus can be used.

Next, the shape of the oxide semiconductor film 733 is processed by aphotolithography method, so that an island-shaped oxide semiconductorfilm 734 and an island-shaped oxide semiconductor film 736 are formed tooverlap with the gate electrode 701 and the gate electrode 702,respectively. Note that a resist mask for forming the island-shapedoxide semiconductor films may be formed by an inkjet method. Formationof the resist mask by an inkjet method needs no photomask; thus,manufacturing cost can be reduced.

In accordance with the steps after the step for forming the sourceelectrode and the drain electrode illustrated in FIG. 11A of Embodiment4, a transistor which serves as a switching element of a memory cell canbe manufactured.

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

Embodiment 8

In this embodiment, a structure of a CPU, one of semiconductor deviceseach of which is one embodiment of the present invention, will bedescribed.

In FIG. 17, the structure of the CPU of this embodiment is illustrated.The CPU shown in FIG. 17 mainly includes an arithmetic logic unit (ALU)901, an ALU controller 902, an instruction decoder 903, an interruptcontroller 904, a timing controller 905, a register 906, a registercontroller 907, a bus interface (Bus I/F) 908, a rewritable ROM 909 anda ROM interface (ROM I/F) 920, over a substrate 900. Further, the ROM909 and the ROM I/F 920 may be provided over another chip. Needless tosay, the CPU shown in FIG. 17 is only an example in which theconfiguration is simplified, and an actual CPU may have variousconfigurations depending on the uses.

An instruction which is input to the CPU through the Bus I/F 908 isinput to the instruction decoder 903 and decoded therein, and then,input to the ALU controller 902, the interrupt controller 904, theregister controller 907, and the timing controller 905.

The ALU controller 902, the interrupt controller 904, the registercontroller 907, and the timing controller 905 conduct various controlsbased on the decoded instruction. Specifically, the ALU controller 902generates signals for controlling the drive of the ALU 901. While theCPU is executing a program, the interrupt controller 904 judges aninterrupt request from an external input/output device or a peripheralcircuit based on its priority or a mask state, and processes therequest. The register controller 907 generates an address of theregister 906, and reads/writes data from/to the register 906 inaccordance with the state of the CPU.

The timing controller 905 generates signals for controlling a drivetiming of the ALU 901, the ALU controller 902, the instruction decoder903, the interrupt controller 904, and the register controller 907. Forexample, the timing controller 905 is provided with an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the clock signal CLK2 to thevarious above circuits.

In the CPU of this embodiment, a memory element having the structuredescribed in any of the above embodiments is provided in the register906. The register controller 907 selects the holding operation in theregister 906 in accordance with an instruction from the ALU 901. Thatis, in the memory element included in the register 906, whether data isheld in a phase-inversion element or a capacitor is selected. In thecase where selection is made so that data is held in the phase-inversionelement, a power supply voltage is applied to the memory element in theregister 906. In the case where selection is made so that data is heldin the capacitor, the data is written into the capacitor, and theapplication of the power supply voltage to the memory element in theregister 906 can be stopped. The power supply can be stopped byproviding a switching element between a memory element group and a nodeto which a power supply potential VDD or a power supply potential VSS issupplied, as illustrated in FIG. 13A or FIG. 13B.

In such a manner, even in the case where the operation of the CPU istemporally stopped and the application of the power supply voltage isstopped, data can be held and power consumption can be reduced.Specifically, for example, while a user of a personal computer does notinput data to an input device such as a keyboard, the operation of theCPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example in this embodiment, thesemiconductor device of the present invention is not limited to the CPU,but can be applied to an LSI such as a DSP, a custom LSI, or a fieldprogrammable gate array (FPGA).

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

Example 1

A semiconductor device which is one embodiment of the present inventionis used, so that a highly reliable electronic device and an electronicdevice with low power consumption can be provided. In particular, in thecase of a portable electronic device which has difficulty incontinuously receiving electric power, a semiconductor device with lowpower consumption which is one embodiment of the present invention isadded to the component of the device, whereby an advantage in increasingthe continuous operation time can be obtained. Further, with the use ofa transistor with low OFF-state current, redundant circuit design whichis needed to cover a failure caused by high OFF-state current isunnecessary; therefore, the density of an integrated circuit used forthe semiconductor device can be increased, and the semiconductor devicecan have a higher performance.

A semiconductor device which is one embodiment of the present inventioncan be used for display devices, personal computers, or imagereproducing devices provided with recording media (typically, deviceswhich reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images). Inaddition, as an electronic device which can include the semiconductordevice which is one embodiment of the present invention, mobile phones,game machines including portable game machines, portable informationterminals, e-book readers, video cameras, digital still cameras,goggle-type displays (head mounted displays), navigation systems, audioreproducing devices (e.g., car audio systems and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), vending machines, and the like can be given. Specificexamples of these electronic devices are shown in FIGS. 18A to 18F.

FIG. 18A illustrates an e-book reader including a housing 7001, adisplay portion 7002, and the like. A semiconductor device which is oneembodiment of the present invention can be used for an integratedcircuit which controls driving of the e-book reader. With the use of thesemiconductor device which is one embodiment of the present inventionfor the integrated circuit which controls driving of the e-book reader,a highly reliable e-book reader and an e-book reader with low powerconsumption can be provided. When a flexible substrate is used, asemiconductor device and a semiconductor display device can haveflexibility, whereby a user-friendly e-book reader which is flexible andlightweight can be provided.

FIG. 18B illustrates a display device including a housing 7011, adisplay portion 7012, a supporting base 7013, and the like. Asemiconductor device which is one embodiment of the present inventioncan be used for an integrated circuit which controls driving of thedisplay device. With the use of the semiconductor device which is oneembodiment of the present invention for the integrated circuit whichcontrols driving of the display device, a highly reliable display deviceand a display device with low power consumption can be provided. Notethat a display device includes all display devices for displayinginformation, such as display devices for personal computers, forreceiving television broadcast, and for displaying advertisement, in itscategory.

FIG. 18C illustrates a display device including a housing 7021, adisplay portion 7022, and the like. A semiconductor device which is oneembodiment of the present invention can be used for an integratedcircuit which controls driving of the display device. With the use ofthe semiconductor device which is one embodiment of the presentinvention for the integrated circuit which controls driving of thedisplay device, a highly reliable display device and a display devicewith low power consumption can be provided. When a flexible substrate isused, a semiconductor device and a semiconductor display device can haveflexibility, whereby a user-friendly display device which is flexibleand lightweight can be provided. Accordingly, as illustrated in FIG.18C, a display device can be used while being fixed to fabric or thelike, and an application range of the semiconductor display device isdramatically widened.

FIG. 18D illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, operation keys 7037, a stylus 7038, andthe like. A semiconductor device which is one embodiment of the presentinvention can be used for an integrated circuit which controls drivingof the portable game machine. With the use of the semiconductor devicewhich is one embodiment of the present invention for the integratedcircuit which controls driving of the portable game machine, a highlyreliable portable game machine and a portable game machine with lowpower consumption can be provided. Although the portable game machineillustrated in FIG. 18D has the two display portions 7033 and 7034, thenumber of display portions included in the portable game machines is notlimited thereto.

FIG. 18E illustrates a mobile phone including a housing 7041, a displayportion 7042, an audio-input portion 7043, an audio-output portion 7044,operation keys 7045, a light-receiving portion 7046, and the like. Lightreceived in the light-receiving portion 7046 is converted intoelectrical signals, whereby external images can be loaded. Asemiconductor device which is one embodiment of the present inventioncan be used for an integrated circuit which controls driving of themobile phone. With the use of the semiconductor device which is oneembodiment of the present invention for the integrated circuit whichcontrols driving of the mobile phone, a highly reliable mobile phone anda mobile phone with low power consumption can be provided.

FIG. 18F illustrates a portable information terminal including a housing7051, a display portion 7052, operation keys 7053, and the like. In theportable information terminal illustrated in FIG. 18F, a modem may beincorporated in the housing 7051. A semiconductor device which is oneembodiment of the present invention can be used for an integratedcircuit which controls driving of the portable information terminal.With the use of the semiconductor device which is one embodiment of thepresent invention for the integrated circuit which controls driving ofthe portable information terminal, a highly reliable portableinformation terminal and a portable information terminal with low powerconsumption can be provided.

Example 1 can be implemented by being combined as appropriate with anyof the above-described embodiments.

This application is based on Japanese Patent Application serial no.2009-293982 filed with Japan Patent Office on Dec. 25, 2009, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

100: memory element, 101: phase-inversion element, 102: phase-inversionelement, 103: switching element, 104: switching element, 105: capacitor,106: capacitor switching element, 107: p-channel transistor, 108:n-channel transistor, 109: p-channel transistor, 110: n-channeltransistor, 200: memory element, 201: phase-inversion element, 202:phase-inversion element, 203: switching element, 204: switching element,205: capacitor, 206: capacitor switching element, 207: phase-inversionelement, 208: switching element, 209: switching element, 210: p-channeltransistor, 211: n-channel transistor, 212: p-channel transistor, 213:n-channel transistor, 214: p-channel transistor, 215: n-channeltransistor, 300: memory element, 301: phase-inversion element, 302:phase-inversion element, 303: switching element, 304: switching element,305: capacitor, 306: capacitor switching element, 307: capacitor, 308:capacitor switching element, 309: p-channel transistor, 310: n-channeltransistor, 311: p-channel transistor, 312: n-channel transistor, 401:switching element, 402: memory element, 403: memory element group, 420:transistor, 421: first electrode, 422: oxide semiconductor film, 423:second electrode, 424: gate insulating film, 425: gate electrode, 426:insulating film, 431: contact hole, 432: contact hole, 433: contacthole, 434: wiring, 435: wiring, 436: wiring, 440: wiring, 441: contacthole, 500: bond substrate, 501: insulating film, 502: embrittlementlayer, 503: base substrate, 504: semiconductor film, 505: semiconductorfilm, 506: semiconductor film, 507: semiconductor film, 508: gateinsulating film, 509: electrode, 510: impurity region, 511: impurityregion, 512: side wall, 513: high-concentration impurity region, 514:low-concentration impurity region, 515: channel formation region, 516:high-concentration impurity region, 517: low-concentration impurityregion, 518: channel formation region, 520: n-channel transistor, 521:p-channel transistor, 530: insulating film, 531: insulating film, 532:insulating film, 601: gate electrode, 602: electrode, 603: gateinsulating film, 605: oxide semiconductor film, 607: source electrode,608: drain electrode, 609: wiring, 610: wiring, 611: wiring, 612:insulating film, 620: transistor, 623: capacitor, 630: transistor, 631:gate electrode, 632: gate insulating film, 633: oxide semiconductorfilm, 634: channel protective film, 635: source electrode, 636: drainelectrode, 637: insulating film, 640: transistor, 641: gate electrode,642: gate insulating film, 643: source electrode, 644: drain electrode,645: oxide semiconductor film, 646: insulating film, 650: transistor,651: source electrode, 652: drain electrode, 653: oxide semiconductorfilm, 654: gate insulating film, 655: gate electrode, 656: insulatingfilm, 660: semiconductor substrate, 661: n-channel transistor, 662:p-channel transistor, 663: insulating film, 664: transistor, 665:capacitor, 666: element isolation insulating film, 701: gate electrode,702: gate electrode, 703: gate insulating film, 730: oxide semiconductorfilm, 731: oxide semiconductor film, 732: oxide semiconductor film, 733:oxide semiconductor film, 734: oxide semiconductor film, 735: oxidesemiconductor film, 736: oxide semiconductor film, 900: substrate, 901:ALU, 902: ALU controller, 903: instruction decoder, 904: interruptcontroller, 905: timing controller, 906: register, 907: registercontroller, 908: bus I/F, 909: ROM, 920: ROM I/F, 1300: register, 1301:inverter, 1302: inverter, 1303: switching element, 1304: switchingelement, 1310: p-channel transistor, 1311: n-channel transistor, 1312:p-channel transistor, 1313: n-channel transistor, 7001: housing, 7002:display portion, 7011: housing, 7012: display portion, 7013: supportingbase, 7021: housing, 7022: display portion, 7031: housing, 7032:housing, 7033: display portion, 7034: display portion, 7035: microphone,7036: speaker, 7037: operation key, 7038: stylus, 7041: housing, 7042:display portion, 7043: audio-input portion, 7044: audio-output portion,7045: operation key, 7046: light-receiving portion, 7051: housing, 7052:display portion, 7053: operation key.

1. A memory device comprising a plurality of memory elements, the memoryelements each comprising: a first phase-inversion element and a secondphase-inversion element which hold data by being connected to each othersuch that an input terminal of the first phase-inversion element isconnected to an output terminal of the second phase-inversion elementand an input terminal of the second phase-inversion element is connectedto an output terminal of the first phase-inversion element; a capacitor;and a transistor which includes an oxide semiconductor in a channelformation region and which is configured to control writing of the datato the capacitor.
 2. A memory device comprising a plurality of memoryelements, the memory elements each comprising: a first phase-inversionelement; a second phase-inversion element an input terminal of which isconnected to an output terminal of the first phase-inversion element; afirst switching element which is configured to control input of a signalincluding data to an input terminal of the first phase-inversionelement; a second switching element which is configured to controlconnection between the input terminal of the first phase-inversionelement and an output terminal of the second phase-inversion element; acapacitor; and a transistor which includes an oxide semiconductor in achannel formation region and which is configured to control writing ofthe data to the capacitor.
 3. A memory device comprising a plurality ofmemory elements, the memory elements each comprising: a firstphase-inversion element; a second phase-inversion element an inputterminal of which is connected to an output terminal of the firstphase-inversion element; a first switching element which is configuredto control input of a signal including data to an input terminal of thefirst phase-inversion element; a second switching element which isconfigured to control connection between the input terminal of the firstphase-inversion element and an output terminal of the secondphase-inversion element; a capacitor; and a transistor which includes anoxide semiconductor in a channel formation region and which isconfigured to control connection between the input terminal of the firstphase-inversion element and the capacitor.
 4. A memory device comprisinga plurality of memory elements, the memory elements each comprising: afirst phase-inversion element; a second phase-inversion element an inputterminal of which is connected to an output terminal of the firstphase-inversion element; a first switching element which is configuredto control input of a signal including data to an input terminal of thefirst phase-inversion element; a second switching element which isconfigured to control connection between the input terminal of the firstphase-inversion element and an output terminal of the secondphase-inversion element; a third switching element which is configuredto control output of a potential of the output terminal of the firstphase-inversion element; a capacitor; a transistor which includes anoxide semiconductor in a channel formation region and which isconfigured to control writing of the data to the capacitor; a thirdphase-inversion element an input terminal of which is supplied with apotential including the data written in the capacitor; and a fourthswitching element which is configured to control output of a potentialof an output terminal of the third phase-inversion element.
 5. A memorydevice comprising a plurality of memory elements, the memory elementseach comprising: a first phase-inversion element; a secondphase-inversion element an input terminal of which is connected to anoutput terminal of the first phase-inversion element; a first switchingelement which is configured to control connection between an inputterminal of the first phase-inversion element and a node to which asignal including data is input; a second switching element which isconfigured to control connection between the input terminal of the firstphase-inversion element and an output terminal of the secondphase-inversion element; a third switching element which is configuredto control output of a potential of the output terminal of the firstphase-inversion element; a capacitor; a transistor which includes anoxide semiconductor in a channel formation region and which isconfigured to control connection between the node and the capacitor; athird phase-inversion element an input terminal of which is suppliedwith a potential including the data written in the capacitor; and afourth switching element which is configured to control output of apotential of an output terminal of the third phase-inversion element. 6.The memory device according to any one of claims 1 to 5, wherein anOFF-state current density of the transistor is less than or equal to 100zA/μm.
 7. A memory device comprising a plurality of memory elements, thememory elements each comprising: a first phase-inversion element; asecond phase-inversion element an input terminal of which is connectedto an output terminal of the first phase-inversion element; a firstswitching element which is configured to control input of a signalincluding data to an input terminal of the first phase-inversionelement; a second switching element which is configured to controlconnection between the input terminal of the first phase-inversionelement and an output terminal of the second phase-inversion element; afirst capacitor; a first transistor which includes an oxidesemiconductor in a channel formation region and which is configured tocontrol writing of the data to the first capacitor; a second capacitor;and a second transistor which includes an oxide semiconductor in achannel formation region and which is configured to control writing ofthe data to the second capacitor.
 8. A memory device comprising aplurality of memory elements, the memory elements each comprising: afirst phase-inversion element; a second phase-inversion element an inputterminal of which is connected to an output terminal of the firstphase-inversion element; a first switching element which is configuredto control input of a signal including data to an input terminal of thefirst phase-inversion element; a second switching element which isconfigured to control connection between the input terminal of the firstphase-inversion element and an output terminal of the secondphase-inversion element; a first capacitor; a first transistor whichincludes an oxide semiconductor in a channel formation region and whichis configured to control connection between the input terminal of thefirst phase-inversion element and the first capacitor; a secondcapacitor; and a second transistor which includes an oxide semiconductorin a channel formation region and which is configured to controlconnection between the output terminal of the first phase-inversionelement and the second capacitor.
 9. The memory device according toclaim 7 or claim 8, wherein an OFF-state current density of the firsttransistor or the second transistor is less than or equal to 100 zA/μm.10. The memory device according to any one of claims 1, 2, 3, 7, and 8,wherein each of the first and second phase-inversion elements is oneselected from the group of an inverter and a clocked inverter.
 11. Thememory device according to any one of claims 1, 2, 3, 4, 5, 7, and 8,wherein the oxide semiconductor is an In—Ga—Zn—O-based oxidesemiconductor.
 12. The memory device according to any one of claims 1,2, 3, 4, 5, 7, and 8, wherein an hydrogen concentration in the channelformation region is lower than or equal to 5×10¹⁹/cm³.
 13. Asemiconductor device which is an LSI comprising a CPU, a DSP, or amicrocontroller comprising the memory device according to any one ofclaims 1, 2, 3, 4, 5, 7, and
 8. 14. An electric device including oneselected from the group consisting of a game machine, an imagereproducing device, and a personal computer each comprising the memorydevice according to any one of claims 1, 2, 3, 4, 5, 7, and
 8. 15. Thememory device according to claim 4 or claim 5, wherein each of thefirst, second and third phase-inversion elements is one selected fromthe group of an inverter and a clocked inverter.